Relay apparatus

ABSTRACT

A method according to one embodiment is a method executed by a relay apparatus configured to: connect to a first upper apparatus; and have a function of relaying communication between the first upper apparatus and a lower apparatus. The method includes: detecting a connection failure with the first upper apparatus after starting the relaying; transmitting a reconnection request to a second upper apparatus which is a candidate of a reconnection destination of the relay apparatus in response to detection of the connection failure; receiving, from the second upper apparatus, in response to transmission of the reconnection request, a radio resource control, RRC, setup message indicating that not the reconnection but an RRC connection establishment has to be performed; and transmitting, to the second upper apparatus, information on behavior of the relay apparatus in response to reception of the RRC setup message.

RELATED APPLICATIONS

The present application is a continuation based on PCT Application No.PCT/JP2019/035620, filed on Sep. 11, 2019, which claims the benefit ofU.S. Provisional Application No. 62/736,575 filed on Sep. 26, 2018. Thecontent of which is incorporated by reference herein in their entirety.

TECHNICAL FIELD

The present disclosure relates to a relay apparatus.

BACKGROUND ART

The 3rd Generation Partnership Project (3GPP), which is astandardization project for mobile communication systems, is studying anew relay apparatus called an Integrated Access and Backhaul (IAB) node.One or a plurality of relay apparatuses are involved in communicationsbetween a base station and user equipment, and relay the communications.Such a relay apparatus has a user equipment function and a base stationfunction, uses the user equipment function to perform radiocommunications with an upper node (base station or upper relayapparatus), and uses the base station function for performing radiocommunications with a lower node (user equipment or lower relayapparatus).

A radio section between the user equipment and the relay apparatus orthe base station may be referred to as an access link. A radio sectionbetween the relay apparatus and the base station or another relayapparatus may be referred to as a backhaul link. 3GPP ContributionRP-170217 describes a method of integrating and multiplexing datacommunications in the access link and data communications in thebackhaul link in Layer 2, and dynamically allocating radio resources tothe backhaul link to dynamically switch the data transfer route.

SUMMARY

A method according to one embodiment is a method executed by a relayapparatus configured to: connect to a first upper apparatus; and have afunction of relaying communication between the first upper apparatus anda lower apparatus. The method includes: detecting a connection failurewith the first upper apparatus after starting the relaying; transmittinga reconnection request to a second upper apparatus which is a candidateof a reconnection destination of the relay apparatus in response todetection of the connection failure; receiving, from the second upperapparatus, in response to transmission of the reconnection request, aradio resource control, RRC, setup message indicating that not thereconnection but an RRC connection establishment has to be performed;and transmitting, to the second upper apparatus, information on behaviorof the relay apparatus in response to reception of the RRC setupmessage.

An apparatus according to one embodiment is an apparatus for controllinga relay apparatus configured to: connect to a first upper apparatus; andhave a function of relaying communication between the first upperapparatus and a lower apparatus, the apparatus comprising a processorand a memory. The processor is configured to execute processes of:detecting a connection failure with the first upper apparatus afterstarting the relaying; transmitting a reconnection request to a secondupper apparatus which is a candidate of a reconnection destination ofthe relay apparatus in response to detection of the connection failure;receiving, from the second upper apparatus, in response to transmissionof the reconnection request, a radio resource control, RRC, setupmessage indicating that not the reconnection but an RRC connectionestablishment has to be performed; and transmitting, to the second upperapparatus, information on behavior of the relay apparatus in response toreception of the RRC setup message.

A relay apparatus according to one embodiment is configured to: connectto a first upper apparatus; and have a function of relayingcommunication between the first upper apparatus and a lower apparatus,the relay apparatus comprising a processor and a memory. The processoris configured to execute processes of: detecting a connection failurewith the first upper apparatus after starting the relaying; transmittinga reconnection request to a second upper apparatus which is a candidateof a reconnection destination of the relay apparatus in response todetection of the connection failure; receiving, from the second upperapparatus, in response to transmission of the reconnection request, aradio resource control, RRC, setup message indicating that not thereconnection but an RRC connection establishment has to be performed;and transmitting, to the second upper apparatus, information on behaviorof the relay apparatus in response to reception of the RRC setupmessage.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a configuration of a mobilecommunication system according to an embodiment.

FIG. 2 is a diagram illustrating a configuration of a base station (gNB)according to the embodiment.

FIG. 3 is a diagram illustrating a configuration of a relay apparatus(IAB node) according to the embodiment.

FIG. 4 is a diagram illustrating a configuration of a user equipment(UE) according to the embodiment.

FIG. 5 is a diagram illustrating an example of a protocol stackconfiguration of a user plane in the mobile communication systemaccording to the embodiment.

FIG. 6 is a diagram illustrating an example of a normal operationsequence according to a first embodiment.

FIG. 7 is a diagram illustrating an example of a table for determining acontext transfer destination according to the first embodiment.

FIG. 8 is a diagram illustrating an example of an exceptional operationsequence according to the first embodiment.

FIG. 9 is a diagram illustrating an example of a multi-hop connectionsequence according to the first embodiment.

FIG. 10 is a diagram illustrating an operation example of the IAB nodeaccording to a second embodiment.

FIG. 11 is a diagram illustrating an operation example according to athird embodiment.

FIG. 12A illustrates an example of creation of a connection relationship(topology) according to the third embodiment,

FIG. 12B illustrates an example of creation of a routing table accordingto the third embodiment.

FIG. 13 is a diagram illustrating an example of handover of the IAB nodein an IAB active state according to the third embodiment.

FIG. 14 is a diagram illustrating an example of an operation environmentaccording to a fifth embodiment.

FIG. 15 is a diagram illustrating an example of an operation environmentaccording to a variation 2 of the fifth embodiment.

FIG. 16 is a diagram illustrating an example of an operation environmentaccording to a variation 3 of the fifth embodiment.

FIG. 17 is a diagram illustrating an example of an operation environmentaccording to a variation 4 of the fifth embodiment.

FIG. 18 is a diagram illustrating an example of an operation environmentaccording to a variation 5 of the fifth embodiment.

FIG. 19 is a diagram illustrating an example of an operation environmentaccording to a variation 6 of the fifth embodiment.

FIG. 20 is a diagram illustrating an example of an operation environmentaccording to a variation 7 of the fifth embodiment.

FIG. 21 is a diagram illustrating an example of an operation environmentaccording to a variation 8 of the fifth embodiment.

FIG. 22 is a diagram illustrating an operation example according to aseventh embodiment.

FIGS. 23A, 23B, and 23C are a diagram illustrating links/routes in IABtopology and DAG.

FIG. 24 is a diagram illustrating an example of topology management androute management.

FIG. 25 is a diagram illustrating an example of a scenario of the IABfailure recover.

DESCRIPTION OF EMBODIMENTS

A mobile communication system according to one embodiment will bedescribed with reference to the accompanying drawings. Note that in thedescriptions of the drawing below, identical or similar symbols areassigned to identical or similar portions.

(A Configuration of a Mobile Communication System)

A configuration of a mobile communication system according to anembodiment will be described. FIG. 1 is a diagram illustrating aconfiguration of a mobile communication system 1 according to anembodiment. The mobile communication system 1 is a 5G mobilecommunication system based on 3GPP standard. In particular, a radioaccess format in the mobile communication system 1 is NR (New Radio)which is a radio access format of 5G. However, LTE (Long Term Evolution)may be applied to the mobile communication system 1 at least partially.

As shown in FIG. 1, the mobile communication system 1 comprises a 5Gcore network (5GC) 10, a user equipment (UE) 100, a base station(referred to as gNB) 200, and an IAB node 300. In an embodiment, anexample in which the base station is an NR base station will be mainlydescribed, but the base station may be an LTE base station (i.e., aneNB).

The 5GC10 comprises an AMF (Access and Mobility Management Function) 11and an UPF (User Plane Function) 12. The AMF 11 is an apparatus whichperforms various mobility control with respect to the UE 100, and thelike. The AMF 11 manages information of an area in which the UE 100exists by using non-access stratum (NAS) signaling to communicate withthe UE 100. The UPF 12 is an apparatus which performs data transfercontrol, and like.

The gNB 200 is connected to the 5GC 10 via an interface referred to asan NG interface. In FIG. 1, three gNB 200-1 to gNB 200-3 connected tothe 5GC 10 are illustrated. The gNB 200 is a fixed radio communicationdevice which performs radio communication with the UE 100. When the gNB200 has a donor function, the gNB 200 may perform radio communicationwith an IAB node which wirelessly connects to itself.

The gNB 200 is connected to another gNB 200 in an adjacent relationshipvia an inter-base station interface referred to as an Xn interface. FIG.1 shows an example in which the gNB200-1 is connected to the gNB200-2and the gNB200-2.

Each of the gNB 200 manages one or a plurality of cells. A cell is usedas a term indicating the smallest unit of a radio communications area. Acell is also used as a term indicating a function or resource whichperforms radio communications with the UE 100. One cell belongs to onecarrier frequency.

The UE 100 is a mobile radio communication device that performs radiocommunication with the gNB 200. The UE 100 may communicate with the IABnode 300. The UE 100 may be any device as long as it is a device thatperforms radio communication with the gNB 200 or the IAB node 300. Forexample, the UE 100 is a mobile phone terminal, a tablet terminal, anotebook PC, a sensor, a device installed in the sensor, a vehicle, or adevice installed in vehicle.

FIG. 1 shows an example in which the UE 100-1 wirelessly connects to thegNB 200-1, the UE 100-2 wirelessly connects to the IAB node 300-1, andthe UE 100-3 wirelessly connects to the IAB 300-2. The UE 100-1 directlycommunicates with the gNB 200-1. The UE 100-2 indirectly communicateswith the gNB 200-1 via the IAB node 300-1. The UE 100-3 indirectlycommunicates with the gNB 200-1 via the IAB node 300-1 and the IAB node300-2.

An IAB node 300 is a device (relay apparatus) that is involved incommunications between an eNB 200 and a UE 100 and relays thecommunications. FIG. 1 illustrates an example in which an IAB node 300-1is in radio connection with a gNB 200-1 serving as a donor and an IABnode 300-2 is in wireless connection with the IAB node 300-1. Each IABnode 300 manages a cell. The cell managed by the IAB node 300 may have acell ID that may be the same as or different from the cell ID of thecell of the donor gNB 200-1.

The IAB node 300 has a UE function (user equipment function) and a gNBfunction (base station function). The IAB node 300 uses the UE functionto perform radio communications with an upper node (the gNB 200 or aupper IAB node 300), and also uses the gNB function to perform radiocommunications with a lower node (UE 100 or lower IAB node 300). The UEfunction is at least a part of the functions of the UE 100, and the IABnode 300 does not necessarily need to have all the functions of the UE100. The gNB function is at least a part of the functions of the gNB200, and the IAB node 300 does not necessarily need to have all thefunctions of the gNB 200.

A radio section between the UE 100 and the IAB node 300 or gNB 200 maybe referred to as an access link (or Uu). A radio section between theIAB node 300 and the gNB 200 or another IAB node 300 may be referred toas a backhaul link (or Un). Such a backhaul link may be referred to as afronthaul link.

A relay route can be dynamically switched by integrating andmultiplexing data communications in the access link and datacommunications in the backhaul link in Layer 2, and dynamicallyallocating radio resources to data communications in the backhaul link.A millimeter wave band may be used for the access link and the backhaullink. In addition, the access link and the backhaul link may be timedivision and/or frequency division multiplexed.

(A Configuration of gNB)

A configuration of the gNB 200 according to an embodiment will bedescribed. FIG. 2 is a diagram illustrating a configuration of the gNB200. As shown in FIG. 2, the gNB 200 comprises a radio communicator 210,a network communicator 220, and a controller 230.

The radio communicator 210 is used for radio communication with the UE100 and radio communication with the IAB node 300. The radiocommunicator 210 includes a receiver 211 and a transmitter 212. Thereceiver 211 performs various receptions under the control of thecontroller 230. The receiver 211 includes an antenna and converts aradio signal received by the antenna into a baseband signal (receivedsignal) and outputs the baseband signal to the controller 230. Thetransmitter 212 performs a variety of transmission under the control ofthe controller 230. The transmitter 212 includes an antenna and convertsthe baseband signal (transmitted signal) output from the controller 230into the radio signal and transmits the radio signal from the antenna.

The network communicator 220 is used for wired communication (orwireless communication) with the 5GC 10 and wired communication (orwireless communication) with another adjacent gNB 200. The networkcommunicator 220 includes a receiver 221 and a transmitter 222. Thereceiver 221 performs various receptions under the control of thecontroller 230. The receiver 221 receives a signal from the outside andoutputs the reception signal to the controller 230. The transmitter 222performs a variety of transmission under the control of the controller230. The transmitter 222 transmits, to the outside, a transmissionsignal output from the controller 230.

The controller 230 performs various controls in the gNB 200. Thecontroller 230 includes at least one processor and memory. The memorystores a program executed by the processor and information used forprocessing by the processor. The processor may include a basebandprocessor and a central processing unit (CPU). The baseband processorperforms modulation/demodulation, encoding/decoding, and the like of thebaseband signal. The CPU performs a variety of processing by executingprograms stored in the memory. The processor executes processing to bedescribed later.

(A Configuration of IAB Node)

A configuration of the IAB node 300 according to an embodiment will bedescribed. FIG. 3 is a diagram illustrating a configuration of the IABnode 300. As shown in FIG. 3, the IAB node 300 comprises a radiocommunicator 310 and a controller 320.

The radio communicator 310 is used for radio communication with the gNB200 (backhaul link) and radio communication with the UE 100 (accesslink). The radio communicator 310 includes a receiver 311 and atransmitter 312. The receiver 311 performs various receptions under thecontrol of the controller 320. The receiver 311 includes an antenna andconverts a radio signal received by the antenna into a baseband signal(received signal) and outputs the baseband signal to the controller 320.The transmitter 312 performs a variety of transmission under the controlof the controller 320. The transmitter 312 includes an antenna andconverts the baseband signal (transmitted signal) output from thecontroller 320 into the radio signal and transmits the radio signal fromthe antenna.

The controller 320 performs various controls in the IAB node 300. Thecontroller 320 includes at least one processor and memory. The memorystores a program executed by the processor and information used forprocessing by the processor. The processor may include a basebandprocessor and a central processing unit (CPU). The baseband processorperforms modulation/demodulation, encoding/decoding, and the like of thebaseband signal. The CPU performs a variety of processing by executingprograms stored in the memory. The processor executes processing to bedescribed later.

(A configuration of UE)

A configuration of the UE 100 according to an embodiment will bedescribed. FIG. 4 is a diagram illustrating a configuration of the UE100. As shown in FIG. 4, the UE 100 comprises a radio communicator 110and a controller 120.

The radio communicator 110 is used for radio communication in accesslink, i.e., radio communication with the gNB 200 and radio communicationwith the IAB node 300. The radio communicator 110 includes a receiver111 and a transmitter 112. The receiver 111 performs various receptionsunder the control of the controller 120. The receiver 111 includes anantenna and converts a radio signal received by the antenna into abaseband signal (received signal) and outputs the baseband signal to thecontroller 120. The transmitter 112 performs a variety of transmissionunder the control of the controller 120. The transmitter 112 includes anantenna and converts the baseband signal (transmitted signal) outputfrom the controller 120 into the radio signal and transmits the radiosignal from the antenna.

The controller 120 performs various controls in the UE 100. Thecontroller 120 includes at least one processor and memory. The memorystores a program executed by the processor and information used forprocessing by the processor. The processor may include a basebandprocessor and a central processing unit (CPU). The baseband processorperforms modulation/demodulation, encoding/decoding, and the like of thebaseband signal. The CPU performs a variety of processing by executingprograms stored in the memory. The processor executes processing to bedescribed later.

(Example of Protocol Stack Configuration)

An example of a protocol stack configuration in the mobile communicationsystem 1 according to an embodiment will be described. FIG. 5 is adiagram illustrating an example of a protocol stack configuration of auser plane. Here, an example of a protocol stack configuration relatedto user data transmission between a UE 100-3 and a UPF 12 in a 5GC 10illustrated in FIG. 1 will be described.

As illustrated in FIG. 5, the UPF 12 includes a GPRS Tunneling Protocolfor User Plane (GTP-U), User Datagram Protocol (UDP), Internet Protocol(IP), and Layer 1/Layer 2 (L1/L2). The gNB 200-1 (donor gNB) is providedwith a protocol stack corresponding to these.

The gNB 200-1 includes a central unit (CU) and a distributed unit (DU).In the protocol stack of the radio interface, Packet Data ConvergenceProtocol (PDCP) and higher layers are provided to the CU, and Radio LinkControl (RLC) and lower layers are provided to the DU. The CU and the DUare connected to each other via an interface referred to as an F1interface.

Specifically, the CU includes Service Data Adaptation Protocol (SDAP),PDCP, IP, and L1/L2. SDAP and PDCP of the CU communicate with SDAP andPDCP of the UE 100 via the DU, the IAB node 300-1, and the IAB node300-2.

The DU includes an RLC, an adaptation layer (Adapt), Medium AccessControl (MAC), and a Physical layer (PHY) in the protocol stack of theradio interface. A protocol stack of these is a protocol stack for agNB. The hierarchical relationship between the adaptation layer and theRLC (S-RLC) may be reversed.

The IAB node 300-1 is provided with a protocol stack ST1 that is for UEand corresponds to these. Furthermore, the IAB node 300-1 is providedwith a protocol stack ST2 for gNB. The protocol stack ST1 and theprotocol stack ST2 each include Layer 2 and lower layers (sublayers).Thus, the IAB node 300-1 is a Layer 2 relay apparatus that relays userdata using Layer 2 and lower layers. The IAB node 300-1 performs relaysthe data without using Layer 3 and higher layers (specifically, PDCP andhigher layers). The IAB node 300-2 has a protocol stack configurationsimilar to that of the IAB node 300-1.

The protocol stack configuration in the user plane is as describedabove. In a control plane on the other hand, the gNB 200-1, the IAB node300-1, the IAB node 300-2, and the UE 100-3 each have Radio ResourceControl (RRC) corresponding to Layer 3.

An RRC connection is established between the RRC of the gNB 200-1 (donorgNB) and the RRC of the IAB node 300-1, and RRC messages are transmittedand received using this RRC connection. An RRC connection is establishedbetween the RRC of the gNB 200-1 and the RRC of the IAB node 300-2, andRRC messages are transmitted and received using this RRC connection.Furthermore, an RRC connection is established between the RRC of the gNB200-1 and the RRC of the UE 100-3, and RRC messages are transmitted andreceived using this RRC connection.

First Embodiment

An operation in the mobile communication system 1 according to a firstembodiment will be described. Specifically, an operation performed in acase where the IAB node 300-1 establishes a radio connection to the gNB200-1 (donor gNB) will be described.

In such a case, the IAB node 300-1 first establishes an access linkconnection (first radio connection) with the gNB 200-1 by using the UEfunction. In other words, the IAB node 300-1 functions as the UE 100 toestablish an access link connection with gNB 200-1. The establishment ofthe access link connection includes establishment of the RRC connection.

Next, the gNB 200-1 transmits to the IAB node 300-1, a message forestablishing a backhaul link connection (second radio connection)between the IAB node 300-1 and the gNB 200-1 for the gNB function of theIAB node 300-1, while maintaining the access link connection. In thefirst embodiment, this message is an RRC Reconfiguration messagetransmitted and received using the RRC connection.

As a result, the backhaul link connection is established between the IABnode 300-1 and the gNB 200-1, whereby backhaul link communicationsbetween the IAB node 300-1 and the gNB 200-1 can be started properly.

The RRC reconfiguration message for establishing the backhaul linkconnection may include configuration information on a bearer (or L2link) forming the backhaul link connection, and a cell ID (specifically,transmission configuration on a reference signal and a synchronizationsignal associated with the cell ID) to be transmitted by the IAB node300-1. Such an RRC reconfiguration message will be hereinafter referredto as an IAB node configuration message.

The IAB node configuration message may include configuration informationon a default bearer (or default link). The default bearer (or defaultlink) is, for example, a bearer (or link) for relaying SIB (SystemInformation Block) and relaying Msg3 from UE.

The IAB node configuration message may include configuration informationon the stack on the side of the donor gNB 200-1 and may optionallyinclude configuration information on the stack on the side of the IABnode 300-2 (or UE 100). The configuration information on the stack onthe side of the IAB node 300-2 (or UE 100) may be configured (inadvance) by an operator (OAM) or a group of configurations implicitlynotified using the SIB of the donor gNB 200-1 may be reused therefor.

The configuration contents in the IAB node configuration messagebasically include all the configurations included in the RRCreconfiguration message, and may also include an RLC configuration (anoperation mode such as Acknowledged Mode (AM)/Unacknowledged Mode(UM)/Transparent Mode (TM), Logical Channel Prioritization (LCP)parameter, and the like), a MAC configuration (such as Buffer StatusReport (BSR)/Timing Advance Group (TAG)/Power Headroom (PHR) parameters,and Discontinues Reception (DRX) configuration), and PHY configuration.

The configuration contents in the IAB node configuration message mayfurther include a configuration of an adaptation layer (such as mapping(routing) configuration and priority configuration of lower side orhigher side logical channel).

Furthermore, the configuration contents in the IAB node configurationmessage may include the (virtual) IP address (that is, the L3 address)of the IAB node 300-1 as appropriate. This is because the F1 protocolstack assumes SCTP over IP to be used for establishing an F1 interfaceon an L2 link for example.

The configuration contents in the IAB node configuration message is notlimited to the configuration information on the NR protocol, but may beconfiguration information on the LTE protocol (RLC, MAC, PHY).

In the first embodiment, the IAB node 300-1 may transmit, to the gNB200-1, an indication indicating that the IAB node 300-1 has the functionof the IAB node (that is, the Layer 2 relay function) beforeestablishment of the backhaul link connection or requests establishmentof the backhaul link connection. This enables the gNB 200-1 to properlyinitiate a procedure for establishing the backhaul link connection.Hereinafter, such an indication will be referred to as an IABindication. The IAB indication may include information indicatingintention or capability for preparing the link protocol stack for the UEfunction in the IAB node 300-1 with LTE, NR, or both.

Note that the IAB node 300-1 may transmit the IAB indication afterestablishing the access link connection with the gNB 200-1, or transmitthe IAB indication during the procedure for establishing the access linkconnection with gNB 200-1.

Furthermore, a condition for enabling the IAB indication to betransmitted to the gNB may include a condition that, from the gNB, theSIB including a donor function identifier indicating the gNB has thedonor function has been received. Under such a condition, the IAB node300-1 transmits the IAB indication to the gNB 200-1 only when the donorfunction identifier is received from the gNB 200-1 by the SIB.

In the first embodiment, when the gNB 200-1 has the donor function forestablishing the backhaul link connection with the IAB node 300-1, thegNB 200-1 that has received the IAB indication from the IAB node 300-1may transmit the IAB node configuration message to the IAB node 300-1.On the other hand, when the gNB 200-1 does not have the donor function,the gNB 200-1 that has received the IAB indication from the IAB node300-1 may transmit a handover request for requesting the handover of theIAB node 300-1 to another gNB, instead of transmitting the IAB nodeconfiguration message to the IAB node 300-1. Preferably, the gNB 200-1has information on another gNB having the donor function, stored inadvance. The gNB 200-1 may acquire the information on the other gNBhaving the donor function from the IAB node 300-1. The IAB node 300-1acquires the information from the 5GC 10 (core network) or checks theSIB (donor function identifier) of the adjacent cell to acquire theinformation on the other gNB (adjacent cell) having the donor function,and notifies the gNB 200-1 of the acquired information. The gNB 200-1transmits a handover request to the other gNB having the donor functionbased on the stored information or the information acquired from the IABnode 300-1. As a result, after the IAB node 300-1 has been handed overto the other gNB, the IAB node 300-1 can establish the backhaul linkconnection with the other gNB. Alternatively, when the gNB 200-1 doesnot have the donor function, the IAB node 300-1 requests the 5GC 10 toperform the handover to the cell (gNB) having the donor function, andthe 5GC 10 may execute processing related to the handover.

In the first embodiment, the gNB 200-1 may transmit a measurementconfiguration for configuring radio measurement to the IAB node 300-1 inresponse to the reception of the IAB indication from the IAB node 300-1.After receiving the measurement configuration from the gNB 200-1, theIAB node 300-1 transmits a measurement report including the result ofthe radio measurement to the gNB 200-1. The gNB 200-1 determines whetherit (gNB 200-1) is the appropriate donor gNB or another gNB is theappropriate donor gNB, based on the measurement report from the IAB node300-1. For example, based on the measurement reports, the gNB 200-1determines that the other gNB is the appropriate donor gNB, when themeasurement result for the other gNB is better than the own measurementresult (for the gNB 200-1) with a difference between these measurementreports exceeding a threshold. Otherwise, the gNB 200-1 determines thatit is the appropriate donor gNB.

Upon determining that it (gNB 200-1) is the appropriate donor gNB 200-1,the gNB 200-1 transmits an IAB node configuration message to the IABnode 300-1. Upon determining that another gNB is the appropriate donorgNB, the gNB 200-1 transmits a handover request for requesting thehandover of the IAB node 300-1 to the other gNB, instead of transmittingthe IAB node configuration message to the IAB node 300-1. Thus, afterthe IAB node 300-1 has been handed over to the other gNB under anexcellent radio condition, the IAB node 300-1 can establish the backhaullink connection with the other gNB.

In the first embodiment, the gNB 200-1 may transmit context informationabout the IAB node 300-1 to another gNB after establishing the backhaullink connection. This context information includes AS layer connectionconfiguration on the radio side (content of RRC reconfiguration), PDU(Protocol Data Unit) session resource configuration on the network side(such as UE ID and session ID, QoS (Quality of Service)/sliceconfiguration of AMF or RAN (Radio Access Network)), and other relatedinformation (such as history information and preference information onbehavior, communications, and the like of the IAB node.

Specifically, the gNB 200-1 transmits the context information about theIAB node 300-1 to another gNB in advance even without determining thatthe IAB node 300-1 is to be handed over to another gNB. Thus, when theradio condition between the gNB 200-1 and the IAB node 300-1deteriorates and thus the IAB node 300-1 reestablishes the radioconnection with another gNB, the reestablishment can be swiftlyimplemented using the context information shared in advance.

Here, the gNB 200-1 preferably holds a table in which the IAB node 300-1is associated with candidates of the donor gNB for the IAB node 300-1.The gNB 200-1 transmits the context information to other gNBs that arethe candidates in the table. This allows the gNB 200-1 to share thecontext information with the other appropriate gNBs.

(1) Example of normal operation sequence

FIG. 6 is a diagram illustrating an example of a normal operationsequence in the mobile communication system 1 according to the firstembodiment.

As illustrated in FIG. 6, in step S101, the IAB node 300-1 establishesan access link connection (RRC connection) with the gNB 200-1 by, forexample, performing a random access procedure for the gNB 200-1. The IABnode 300-1 may provide the IAB indication in a message (e.g., Msg3)transmitted to the gNB 200-1 during the random access procedure. The gNB200-1 acquires context information about the IAB node 300-1 in stepS101.

In step S102, the IAB node 300-1 performs an attach procedure to the 5GC10 (specifically, AMF 11) via the gNB 200-1. Here, the IAB node 300-1may notify the AMF 11 of a notification such as the IAB indication (thatis, a notification indicating an intention to function as an IAB node).Thus, the IAB node 300-1 may acquire the list of candidate donor gNBs(cells), routing information indicating presence/absence of a lowernode, other management information, and the like from the AMF 11.

Alternatively, context information such as information indicating thatthe IAB node 300-1 is attached to each candidate donor gNB and routinginformation of the IAB node 300-1 may be notified from the AMF 11. Ifthe IAB node 300-1 is already attached, the attach processing in stepS102 can be omitted. Specifically, the attach processing by the IAB node300-1 is omitted when the connection with the donor gNB needs to bereestablished due to a certain error, as in the RRC reestablishment instep S101.

In step S103, the IAB node 300-1 transmits the IAB indication to the gNB200-1. The transmission of the IAB indication by the IAB node 300-1 maybe triggered by satisfaction of one or a plurality of the followingevents.

-   -   When Msg5 (RRC Complete) is transmitted.    -   When the connection with gNB is established (this may be Msg5        and after, for example, when the first RRC reconfiguration is        done).    -   When IAB configuration information (refer to the above        description) is acquired from the AMF (including a case where        IAB configuration information is already held).    -   Simply when there is an intention to function as an IAB node        (including receiving an instruction to function as an IAB node        from a higher layer).    -   When request to function as an IAB node is received from a lower        IAB node 300-2 or UE 100-3 (when a signal indicating such a        request is received from the lower IAB node 300-2 or the UE        100-3).    -   When the lower IAB node 300-2 or the UE 100-3 is already        connected.

The IAB node 300-1 transmits the RRC message including the IABindication to the gNB 200-1, for example. Such an RRC message may be a“UE Capability Information” message indicating the capability tofunction as a UE. Note that if the IAB indication is transmitted in stepS101, step S103 can be omitted.

Alternatively, the AMF 11 may notify the gNB 200-1 of the IAB indicationin a form of a change in the PDU session resource. The AMF may be an AMFfor IAB management (dedicated).

The description on this normal operation sequence is given under anassumption that the gNB 200-1 has the donor capability. The gNB 200-1determines that the backhaul link connection needs to be establishedwith the IAB node 300-1, based on the IAB indication.

In step S104, the gNB 200-1 transmits the measurement configuration forconfiguring the radio measurement, to the IAB node 300-1. The IAB node300-1 performs the radio measurement based on the measurementconfiguration. For example, the IAB node 300-1 measures the receivedpower (received power of the cell-specific reference signal) for thecell of the current serving cell gNB 200-1 and the cell of the adjacentcell gNB 200-2.

In step S105, IAB node 300-1 transmits a measurement report includingthe results of the radio measurement to the gNB 200-1. The gNB 200-1determines whether it (gNB 200-1) is the appropriate donor gNB oranother gNB is the appropriate donor gNB, based on the measurementreport. Here, the description proceeds under an assumption that the gNB200-1 has determined that it (gNB 200-1) is the appropriate donor gNB.The processing of steps S104 and S105 is not essential and may beomitted.

In step S106, the gNB 200-1 transmits an IAB node configuration message(RRC reconfiguration message) to the IAB node 300-1. The IAB nodeconfiguration message may include a handover instruction designating thecell of the gNB 200-1 (i.e., the current serving cell of the IAB node300-1) as the handover destination. The IAB node 300-1 executesprocessing of establishing a backhaul link connection with the gNB 200-1based on the IAB node configuration message. Such establishmentprocessing includes processing of generating a protocol stack(adaptation/RLC/MAC/PHY entity) for the backhaul link and configuringparameters based on the configuration information in the IAB nodeconfiguration message. Such establishment processing may includeprocessing of preparing a protocol stack on the UE side (for an accesslink) and starting transmission of a synchronization signal or acell-specific reference signal (or processing of preparing for thestart).

In step S107, the IAB node 300-1 transmits a completion notificationmessage to the gNB 200-1. The message indicates that the IAB nodeconfiguration, including the establishment of the backhaul linkconnection, has been completed. After step S107, the IAB node 300-1functions as an IAB node instead of functioning as a UE, for the gNB200-1.

In step S108, the gNB 200-1 transfers the context information acquiredin step S101 to the gNB 200-2 over an Xn interface. The gNB 200-1 holdsa table that associates the IAB node 300-1 with the candidate of thedonor gNB of the IAB node 300-1, and determines the context transferdestination by referring to this table. With the gNB 200-1 thustransferring the context to other gNBs in advance, when the condition ofthe radio connection with the gNB connected to the IAB node 300-1deteriorates, reconnection with the other gNB can be establishedimmediately. FIG. 7 is a diagram illustrating an example of a table fordetermining the context transfer destination. Such a table is configuredin advance for each gNB by, for example, an operator. As illustrated inFIG. 7, in the table, each IAB node is associated with candidates of thedonor gNB for the IAB node. Specifically, each identifier related to theIAB node is associated with an identifier of a candidate of the donorgNB for the IAB node. For example, a gNB geographically close to an IABnode is configured to be a candidate of the donor gNB for that IAB node.Although an example of association with the gNB is described,association with cell IDs may be made. The cell ID may be a physicallayer cell ID or a global cell ID. In addition, the gNB 200-1 maydetermine a gNB 200-1 geographically close to IAB node 300-1 as a donorcandidate, based on the measurement report received from the IAB node300-1. The gNB 200-1 may generate a table in which the IAB node 300-1 isassociated with the candidate of the donor gNB for the IAB node 300-1 orupdate an existing such table, based on the determined donor candidate.

In step S109, the gNB 200-1 transmits a notification to the 5GC 10,indicating that the backhaul link connection with the IAB node 300-1 hasbeen established. Alternatively, the gNB 200-1 may transmit a request toestablish a PDU session for the IAB node to the 5GC 10. As describedabove, the PDU session establishment request may be transmitted from theAMF 11 to the gNB 200-1 before step S109 or in step S109.

(2) Example of Exceptional Operation Sequence

FIG. 8 is a diagram illustrating an example of an exceptional operationsequence in the mobile communication system 1 according to the firstembodiment. In the exceptional operation sequence, the gNB 200-1implements handover of the IAB node 300-1 to the gNB 200-2.

As illustrated in FIG. 8, in step S201, the IAB node 300-1 establishesan access link connection (RRC connection) with the gNB 200-1 by, forexample, performing a random access procedure for the gNB 200-1. The IABnode 300-1 may provide the IAB indication in a message (e.g., Msg3)transmitted to the gNB 200-1 during the random access procedure. The gNB200-1 acquires context information about the IAB node 300-1 in stepS201.

In step S202, the IAB node 300-1 performs an attach procedure to the 5GC10 (specifically, AMF 11) via the gNB 200-1.

In step S203, the IAB node 300-1 transmits the IAB indication to the gNB200-1. The IAB node 300-1 transmits the RRC message including the IABindication to the gNB 200-1, for example. Such an RRC message may be a“UE Capability Information” message indicating the capability of a UE.Note that if the IAB indication is transmitted in step S201, step S203can be omitted.

In step S204, the gNB 200-1 determines whether it has the donorcapability. If the gNB 200-1 does not have the donor capability (stepS204: NO), gNB 200-1 advances the processing to step S208.

If the gNB 200-1 has the donor capability (step S204: YES), in stepS205, the gNB 200-1 transmits the measurement configuration forconfiguring the radio measurement, to the IAB node 300-1. The IAB node300-1 performs the radio measurement based on the measurementconfiguration. For example, the IAB node 300-1 measures the receivedpower (received power of the cell-specific reference signal) for thecell of the current serving cell gNB 200-1 and the cell of the adjacentcell gNB 200-2.

In step S206, the IAB node 300-1 transmits a measurement reportincluding the results of the radio measurement to the gNB 200-1.

In step S207, the gNB 200-1 determines whether it (gNB 200-1) is theappropriate donor gNB or another gNB is the appropriate donor gNB, basedon the measurement report. Upon determining that it (gNB 200-1) is theappropriate donor gNB (step S207: YES), the gNB 200-1 advances theprocessing to step S106 in the above-mentioned normal operation sequence(see FIG. 6).

On the other hand, upon determining that another gNB is the appropriatedonor gNB (step S207: NO), the gNB 200-1 advances the processing to stepS208.

In step S208, the gNB 200-1 transfers the handover request messageincluding the IAB indication received from the IAB node 300-1, to thegNB 200-2 over the Xn interface. The gNB 200-1 may provide the contextinformation acquired in step S201, in the handover request message.Alternatively, the gNB 200-1 may transmit the handover request messageincluding information indicating a request for the IAB node 300-1 tofunction as the donor gNB for the gNB, instead of including the IABindication. In step S208, the gNB 200-1 may transfer the handoverrequest message to the gNB 200-2 over the Xn interface, upon determiningthat the gNB 200-2 has the donor capability. Specifically, for example,when the gNB 200-1 determines that gNB 200-2 is associated with IAB node300-1 as a donor candidate based on the table illustrated in FIG. 7, thegNB 200-1 may transfer the handover request message to the gNB 200-2. Inthis case, the gNB 200-2 is less likely to reject the handover request,whereby the IAB node 300-1 can be handed over more swiftly.Alternatively, information about own donor capability may be shared inadvance between a plurality of gNB 200 s adjacent to each other, via theXn interface. Thus, the gNB 200-1 can identify the adjacent gNB 200having the donor capability, and can transfer the handover requestmessage to the adjacent gNB 200 thus identified.

The gNB 200-2 determines whether to accept the handover of the IAB node300-1, while taking the IAB indication included in the handover requestmessage into consideration. The gNB 200-2 may reject the handoverrequest if it does not have the donor capability. Here, the descriptionwill be given under an assumption that the gNB 200-2 has determined toaccept the handover of the IAB node 300-1.

In step S209, the gNB 200-2 transmits a handover acknowledgment responsemessage to the gNB 200-1 over the Xn interface.

In step S210, the gNB 200-1 transmits a handover instruction message(RRC reconfiguration message) to the IAB node 300-1, based on thehandover acknowledgment response message from the gNB 200-2. Thehandover instruction message includes information for designating the (acell of gNB 200-2) of the handover destination gNB 200-2.

In step S211, the IAB node 300-1 performs handover to the gNB 200-2based on the handover instruction message from the gNB 200.

(3) Example of a Multi-Hop Connection Sequence

FIG. 9 is a diagram illustrating an example of a multi-hop connectionsequence in the mobile communication system 1 according to the firstembodiment. The multi-hop connection sequence is a sequence for the IABnode 300-2 or the UE 100-2 to be connected to the IAB node 300-1 afterthe backhaul link connection has been connected between the IAB node300-1 and gNB 200-1. Here, a case where the IAB node 300-2 is connectedto the IAB node 300-1 will be mainly described, but the IAB node 300-2may be replaced with the UE 100-2 as appropriate. Furthermore, thedescription that has already been made in the above-mentioned “(1)Normal operation sequence” will be omitted.

As illustrated in FIG. 9, in step S301, the IAB node 300-2 establishesan access link connection (RRC connection) with the gNB 200-1 byperforming a random access procedure for the gNB 200-1 via the IAB node300-1. The IAB node 300-2 may transmit a message (e.g., Msg3) includingthe IAB indication to the gNB 200-1 during the random access procedure.The gNB 200-1 acquires context information about the IAB node 300-2 instep S301.

In step S302, the IAB node 300-2 performs an attach procedure to the 5GC10 (specifically, AMF 11) via the IAB node 300-2 and the gNB 200-1.Here, the IAB node 300-2 may notify the AMF 11 of a notification such asthe IAB indication (that is, a notification indicating an intention tofunction as an IAB node). Thus, the IAB node 300-2 may acquire the listof candidate donor gNBs (cells), routing information indicatingpresence/absence of a lower node, other management information, and thelike from the AMF 11. Alternatively, context information such asinformation indicating that the IAB node 300-2 is attached to eachcandidate donor gNB and routing information of the IAB node 300-2 may benotified from the AMF 11. If the IAB node 300-2 is already attached, theattach processing in step S302 can be omitted. Specifically, the attachprocessing by the IAB node 300-2 is omitted when the connection with thedonor gNB needs to be reestablished due to a certain error, as in theRRC reestablishment.

In step S303, the IAB node 300-2 transmits the IAB indication to the gNB200-1 via the IAB node 300-1. What triggers the transmission of the IABindication by the IAB node 300-2 may be similar to that described instep S103 of “(1) Normal operation sequence” described above.

The IAB node 300-2 transmits the RRC message including the IABindication to the gNB 200-1, for example. Such an RRC message may be a“UE Capability Information” message indicating the capability tofunction as a UE. Note that if the IAB indication is transmitted in stepS301, step S303 can be omitted.

Alternatively, the AMF 11 may notify the gNB 200-1 of the IAB indicationin a form of a change in the PDU session resource. The AMF may be an AMFfor IAB management (dedicated).

Since the gNB 200-1 is assumed to have the donor capability in thisoperation sequence, the gNB 200-1 determines that the backhaul linkconnection needs to be established between the IAB node 300-1 and IABnode 300-2 based on the IAB indication.

In step S304, the gNB 200-1 transmits the measurement configuration forconfiguring the radio measurement, to the IAB node 300-2. The IAB node300-2 performs the radio measurement based on the measurementconfiguration.

In step S305, the IAB node 300-2 transmits a measurement reportincluding the results of the radio measurement to the gNB 200-1 via theIAB node 300-1. The gNB 200-1 determines whether it (gNB 200-1) is theappropriate donor gNB or another gNB is the appropriate donor gNB, basedon the measurement report. Here, the description proceeds under anassumption that the gNB 200-1 has determined that it (gNB 200-1) is theappropriate donor gNB. The processing of steps S304 and S305 is notessential and may be omitted.

In step S306, the gNB 200-1 transmits an IAB node configuration message(RRC reconfiguration message) to the IAB node 300-2. The IAB node 300-2executes processing of establishing a backhaul link connection with theIAB node 300-1 based on the IAB node configuration message. Suchestablishment processing includes processing of generating a protocolstack (adaptation/RLC/MAC/PHY entity) for the backhaul link andconfiguring parameters based on the configuration information in the IABnode configuration message. Such establishment processing may includeprocessing of preparing a protocol stack on the UE side (for an accesslink) and starting transmission of a synchronization signal or acell-specific reference signal (or processing of preparing for thestart).

In step S307, the gNB 200-1 transmits an RRC reconfiguration message tothe IAB node 300-1. Such an RRC reconfiguration message is a message forchanging the configuration in the IAB node 300-1 due to the addition ofthe IAB node 300-2. Such an RRC reconfiguration message includes, forexample, mapping information indicating association between a logicalchannel of the IAB node 300-2 and a logical channel of the backhaul linkof the IAB node 300-1. Note that step S307 may be executed before stepS306 or concurrently with step S306.

In step S308, the IAB node 300-2 transmits a completion notificationmessage to the gNB 200-1. The message indicates that the IAB nodeconfiguration, including the establishment of the backhaul linkconnection with the IAB 300-1, has been completed. After step S308, theIAB node 300-2 functions as an IAB node instead of functioning as a UE,for the gNB 200-1.

In step S309, the IAB node 300-1 transmits a completion notificationmessage to the gNB 200-1. The message indicates that the configurationchange due to the establishment of the backhaul link connection with theIAB 300-2, has been completed. Note that step S309 may be executedbefore step S308 or concurrently with step S308.

In step S310, the gNB 200-1 transfers the context information on the IABnode 300-2 acquired in step S301 to the gNB 200-2 over the Xn interface.

In step S311, the gNB 200-1 transmits a notification to the 5GC 10,indicating that the backhaul link connection with the IAB node 300-2 hasbeen established. Alternatively, the gNB 200-1 may transmit a request toestablish a PDU session for the IAB node 300-2 to the 5GC 10. Asdescribed above, the PDU session establishment request may betransmitted from the AMF 11 to the gNB 200-1 before step S311 or in stepS311.

[Variation 1 of the First Embodiment]

In the example described in the above first embodiment, the IAB node300-1 is handed over to the gNB 200-1 due to the gNB 200-1 not havingthe donor capability after the IAB node 300-1 has established a radioconnection with the gNB 200-1. In such an example, each gNB 200 mayprovide information indicating whether it has the donor capability tothe IAB node 300-1. This enables the IAB node 300-1 to select the gNB200 having the donor capability and establish the connection with such agNB 200. For example, the gNB 200 having the donor capability broadcastsa system information block (SIB) including information indicating thatit has the donor capability. The IAB node 300-1 selects the gNB 200 tobe the connection destination, based on the SIB. When the gNB 200 hasthe donor capability and the received power from this gNB 200 does notfall below a threshold, the IAB node 300-1 may select this gNB 200 asthe connection destination. Alternatively, when the gNB 200 does nothave donor capability, the IAB node 300-1 may reselect, upon receivingthe SIB transmitted from the gNB 200, another gNB 200. Then, when theSIB transmitted from the other gNB 200 indicates that the other gNB 200has the donor capability, the IAB node 300-1 performs the random accessprocedure for the other gNB 200 regarded as the connection destinationand may transmit the IAB indication.

Alternatively, each gNB 200, in addition to notifying by the SIB thatthe gNB 200 has the donor ability (donor function), or instead ofnotifying by the SIB that the gNB 200 has the donor ability, may notifyby the SIB that the gNB 200 itself has the ability of handling the IABnode 300. For example, each gNB 200 may notify by the SIB that the gNB200 itself has a function of performing handover of the IAB node 300 toanother gNB (donor gNB).

In the first embodiment described above, an example of including the IABindication in the message (for example, Msg3) sent by the IAB node 300to the gNB 200 during the random access procedure is described. Here,Msg3 is, for example, an RRC Setup Request message. Further, the IABnode 300 may include the IAB indication in Establishment Cause, which isa field (information element) in Msg3.

Alternatively, the IAB node 300 may use the random access preamble(Msg1) sent to the gNB 200 during the random access procedure to notifythe IAB indication. For example, in a case where the SIB givesnotification of the Physical Random Access Channel (PRACH) resources forthe IAB indication, the IAB node 300 may notify the IAB indication bytransmitting the random access preamble using the PRACH resourcesselected from the notified PRACH resources for the IAB indication. Here,the PRACH resources may be time/frequency resources or a signal sequence(preamble sequence).

Alternatively, the IAB node 300 may notify the IAB indication at atiming other than the random access procedure. For example, the IABindication may be included in RRC messages such as a UE AssistanceInformation message.

In the first embodiment described above, the example in which the gNB200 transmits the measurement configurations for configuring the radiomeasurement to the IAB node 300 or the UE 100 and receives a measurementreport including the result of the radio measurement, so that whetherthe gNB 200 itself is an appropriate donor or another gNB is anappropriate donor gNB is determined based on the measurement report isdescribed. However, without limitation to the case where the measurementresult is used at the time of such initial connection, the measurementreport may be used for changing the network topology or changing thedata transfer route.

Second Embodiment

The second embodiment will be described based mainly on a differencefrom the first embodiment described above. The second embodiment may beimplemented in combination with the first embodiment described above anda variation of the first embodiment.

A communication control method according to the second embodiment is amethod in a mobile communication system in which a data transfer routevia at least one IAB node 300 is configured between the donor gNB 200and the UE 100. The communication control method according to the secondembodiment activates the timer in a case where the establishment of aradio connection of the IAB node 300 with the upper apparatus isrejected or the activation of the relay function of the IAB node 300 isdenied by the upper apparatus, or in a case where the radio connectionis released by the upper apparatus. Here, the upper apparatus is anotherIAB node (upper IAB node) under the donor gNB 200 or the donor gNB 200.The upper apparatus may be an apparatus that has an RRC connection withthe IAB node 300 (that is, an apparatus that has an RRC layer). Thetimer specifies the time during which establishment of a radioconnection with the upper apparatus or notification indicating theintention of activating the relay function with the upper apparatusneeds to be avoided. The notification indicating the intention ofactivating the relay function is the above-mentioned IAB indication.

In a case of finding another upper apparatus (for example, another donorgNB or IAB node) that corresponds the relay function during a periodfrom the activation of the timer to the expiration of the timer, the IABnode 300 may establish the radio connection with the another upperapparatus and notify the another upper apparatus of the intention toactivate the relay function. The another upper apparatus thatcorresponds the relay function may be an upper apparatus that transmitsan SIB indicating that the apparatus has the donor function, or an upperapparatus that transmits an SIB indicating that the apparatus has theability to handle an IAB node.

In the operation pattern 1 according to the second embodiment, the IABnode 300 notifies an upper apparatus of the intention (IAB indication)of activating the relay function when establishing a radio connectionwith the upper apparatus (that is, during a random access procedure).For example, the IAB node 300 uses Msg1 or Msg3 of the random accessprocedure to make the notification. The IAB node 300 receives aconnection reject message for rejecting establishment of a radioconnection from the upper apparatus. The connection reject message maybe an RRC Connection Reject message. The IAB node 300 activates thetimer in response to receiving of a connection reject message includinginformation indicating that activation of the relay function is notpossible (for example, “IAB connection unavailable”). The information(“IAB connection unavailable”) may be included in the informationelement “Cause” in the connection reject message. After the timer isactivated, the IAB node 300 does not attempt to establish a radioconnection with the upper apparatus until the timer expires. Forexample, the IAB node 300 does not initiate a random access procedurefor the upper apparatus until the timer expires. The IAB node 300 may beconsidered to be in a state where access to the upper apparatus isprohibited. After the timer expires, the IAB node 300 may attempt toestablish a radio connection with the upper apparatus. Note that, in acase where the IAB node 300 does not intend to activate the relayfunction but desires to establish a radio connection with the upperapparatus, the IAB node 300 may attempt to establish a radio connectionwith the upper apparatus even before the timer expires. In that case,the IAB node 300 does not notify the upper apparatus of the intention(IAB indication) of activating the relay function.

In the operation pattern 2 according to the second embodiment, the IABnode 300 notifies an upper apparatus of the intention (IAB indication)of activating the relay function after establishing a radio connectionwith the upper apparatus (that is, after a random access procedure). Forexample, the IAB node 300 makes the notification by including the IABindication in the RRC message such as the UE Assistance Informationmessage. The IAB node 300 receives a connection release message forreleasing a radio connection from the upper apparatus. The connectionrelease message may be an RRC Connection Release message. The IAB node300 activates the timer in response to receiving of a connection releasemessage including information indicating that activation of the relayfunction is not possible (for example, “IAB connection unavailable”).The information (“IAB connection unavailable”) may be included in theinformation element “Cause” in the connection release message. After thetimer is activated, the IAB node 300 does not attempt to establish aradio connection with the upper apparatus until the timer expires. Forexample, the IAB node 300 does not initiate a random access procedurefor the upper apparatus until the timer expires. The IAB node 300 may beconsidered to be in a state where access to the upper apparatus isprohibited. After the timer expires, the IAB node 300 may attempt toestablish a radio connection with the upper apparatus.

In the operation pattern 3 according to the second embodiment, the IABnode 300 notifies an upper apparatus of the intention (IAB indication)of activating the relay function when establishing a radio connectionwith the upper apparatus, or after establishing a radio connection withthe upper apparatus. The IAB node 300 receives a reconfiguration message(RRC message) including information indicating rejection to activate therelay function (for example, “IAB connection unavailable”) from theupper apparatus. The reconfiguration message may be an RRCReconfiguration message. In this operation pattern, the IAB node 300 mayestablish or maintain a radio connection with the upper apparatus evenif activation of the relay function is rejected by the upper apparatus.The IAB node 300 activates the timer in response to receiving of areconfiguration message including information indicating rejection toactivate the relay function. After the timer is activated, the IAB node300 does not send an IAB indication to the upper apparatus until thetimer expires. After the timer expires, the IAB node 300 may send an IABindication to the upper apparatus. Note that, in a case where the IABnode 300 releases a radio connection with the upper apparatus (in a casewhere the IAB node 300 receives a connection release message (RRCConnection Release message) from the upper apparatus) by the time thetimer expires, the IAB node 300 may stop, abandon, or release the timer.

In the second embodiment, the timer may be configured from the upperapparatus (for example, the donor gNB 200) to the IAB node 300. Thetimer may be configured by the SIB, may be configured by the connectionreject message in the operation pattern 1, may be configured by theconnection release message in the operation pattern 2, or may beconfigured by the reconfiguration message in the operation pattern 3.Alternatively, the timer may be configured in advance in the IAB node300.

Time set to the timers according to the operation patterns 1 and 2according to the second embodiment (hereinafter referred to as “timerfor IAB node”) may be longer than time set to a timer used by the UE 100when a connection is rejected or released (hereinafter referred to as“timer for UE”). The “timer for UE” is a timer included in theconnection reject message and connection release message, and may betime during which the UE 100 needs to wait until an attempt to establisha connection can be made again after receiving the messages (forexample, Wait time). For example, in a case where the IAB node 300establishes a radio connection with another IAB node, it is assumed thatthere may be no connection (backhaul link) between the another IAB nodeand the donor gNB 200. In such a case, the IAB node 300 cannot activatethe relay function unless a connection (backhaul link) is establishedbetween the another IAB node and the donor gNB 200. Therefore, using thetimer for IAB node can reduce the frequency of failure of an attempt toestablish a radio connection by the IAB node 300.

FIG. 10 is a diagram illustrating an operation example of the IAB node300 according to the second embodiment. Here, the operation patterns 1and 2 according to the second embodiment are assumed.

As illustrated in FIG. 10, in Step S401, the IAB node 300 transmits IABindication to an upper apparatus when establishing a radio connectionwith the upper apparatus or after establishing a radio connection withthe upper apparatus. In a case where a connection is not rejected orreleased by the upper apparatus after sending the IAB indication (StepS402: NO), in Step S403, the IAB node 300 activates the relay functionand establishes a backhaul link with the upper apparatus like in theoperation of the first embodiment described above.

In a case where a connection is rejected or released by the upperapparatus after sending the IAB indication (Step S402: YES), that is, ina case where the IAB node 300 receives a connection reject message orconnection release message from the upper apparatus, in Step S404, theIAB node 300 checks whether or not “Cause” in the message is “IABconnection unavailable”. In a case where “Cause” in the message is not“IAB connection unavailable” (Step S404: NO), in Step S405, the IAB node300 activates the timer for UE. Note that, in Step S405, the IAB node300 does not activate the timer for IAB node. Further, in a case whereStep S404 is NO, the timer for IAB node does not need to be included inthe connection reject message or the connection release message, and,even if included, the timer is not activated. After that, the IAB node300 executes the same operation as a normal UE.

In a case where “Cause” in the message is “IAB connection unavailable”(Step S404: YES), in Step S406, the IAB node 300 activates the timer forIAB node. The IAB node 300 does not attempt to establish a radioconnection with the upper apparatus while the timer is operating. TheIAB node 300 may search for another upper apparatus (for example,another donor gNB or IAB node) that corresponds the relay function whilethe timer is operating.

In Step S407, the IAB node 300 checks whether or not another upperapparatus that corresponds the relay function is found. In a case whereanother upper apparatus that corresponds the relay function is found(Step S407: YES), in Step S408, the IAB node 300 stops the timer for IABand attempts to establish a radio connection with the another upperapparatus. Here, the IAB node 300 may transmit the IAB indication to theanother upper apparatus. In this case, the IAB node 300 finds anotherupper apparatus by the same operation as the cell selection of an UE,waits for the another upper apparatus, and then attempts to establish aradio connection with the another upper apparatus.

In a case where another upper apparatus that corresponds the relayfunction is not found (Step S407: NO), in Step S409, the IAB node 300checks whether or not the timer for IAB node has expired. In a casewhere the timer for IAB node has not expired (Step S409: NO), theprocessing returns to Step S407. Note that, in a case where there is noother suitable upper apparatus (for example, another donor gNB 200),after a radio connection with an upper apparatus is established by areattempt to establish a connection in Step S410 described later, theIAB node 300 may notify the network (the donor gNB 200 or 5GC10) of thefact.

In a case where the timer for IAB node has expired (Step S409: YES), inStep S410, the IAB node 300 attempts to establish a radio connectionagain with the upper apparatus that rejects or releases the connectionin Step S402. In a case where the establishment of the radio connectionfails, the IAB node 300 may activate the same timer for IAB node againor activate a longer timer for IAB node.

[Variation of the Second Embodiment]

In the IAB node 300 according to the second embodiment described above,an AS layer that receives rejection of connection, release ofconnection, or rejection of activation of the relay function from theupper apparatus may give notification to a layer higher than the ASlayer. The AS layer includes a PHY layer, a MAC layer, an RLC layer, aPDCP layer, an SDAP layer, and an RRC layer. Here, an RRC layer ismainly assumed. The upper layer is a NAS layer, an application layer, orthe like.

For example, the upper layer of the IAB node 300 may make a transitionto a command waiting mode from an Operation Administration Maintenance(OAM) apparatus of the network operator. The upper layer may stopinstructing the AS layer to establish a connection as an IAB node andinstruct the AS layer to establish a connection as a UE. The upper layermay send error information (which may include failure details,occurrence location information, and occurrence time information) to theOAM apparatus.

Third Embodiment

The third embodiment will be described based mainly on a difference fromthe first embodiment and the second embodiment described above. Thethird embodiment may be implemented in combination with at least one ofthe first and second embodiments described above.

A communication control method according to the third embodiment is amethod in a mobile communication system in which a data transfer routevia at least one IAB node 300 is configured between the donor gNB 200and the UE 100. Each of a plurality of the IAB nodes 300 under the donorgNB 200 notifies an upper apparatus of at least one of informationindicating a radio state of the own IAB node 300, information indicatinga load state of the own IAB node 300, and information indicating acommunication delay state of the own IAB node 300. Here, the upperapparatus is another IAB node (upper IAB node) under the donor gNB 200or the donor gNB 200. The upper apparatus may be an apparatus that hasan RRC connection with the IAB node 300 (that is, an apparatus that hasan RRC layer). In the third embodiment, an example in which the upperapparatus is the donor gNB 200 will be mainly described. The donor gNB200 manages or changes at least one of a connection relationship and adata transfer route in a plurality of the IAB nodes 300 based on theinformation notified from each of a plurality of the IAB nodes 300 underthe donor gNB 200.

FIG. 11 is a diagram illustrating an operation example according to thethird embodiment.

As illustrated in FIG. 11, the IAB node (IAB node #1) 300-1 is connectedby radio to the donor gNB (IAB donor) 200, and the IAB node (IAB node#2) 300-2 and the UE 100 are connected by radio to the IAB node 300-1.An F1 interface, which is a network interface for fronthaul, may beconfigured between the donor gNB 200 and the IAB node 300-1. An F1interface may also be configured between the donor gNB 200 and the IABnode 300-2.

The IAB node 300-1 has an RRC layer, and an RRC message is sent andreceived between an RRC layer of the IAB node 300-1 and an RRC layer ofthe donor gNB 200. Further, the IAB node 300-2 has an RRC layer, and anRRC message is sent and received via the IAB node 300-1 between an RRClayer of the IAB node 300-2 and an RRC layer of the donor gNB 200. TheseRRC messages may be sent and received on the F1 interface.Alternatively, instead of the RRC message, an F1 message specified bythe F1 interface may be sent and received on the F1 interface.Alternatively, an Xn interface may be used. In a case where the Xninterface is used, “F1 interface” below is read as “Xn interface”.Furthermore, the UE 100 has an RRC layer, and an RRC message is sent andreceived via the IAB node 300-1 between an RRC layer of the UE 100 andan RRC layer of the donor gNB 200.

The RRC message and/or F1 message sent to the donor gNB 200 may includea Measurement Report message indicating a measured radio state. Forexample, each of the IAB node 300-1, the IAB node 300-2, and the UE 100performs measurement of a radio state (for example, measurement ofReference Signal Received Power (RSRP), which is receiving power of areference signal, and/or Reference Signal Received Quality (RSRQ), whichis receiving quality of a reference signal) and sends a measurementreport message including a measurement result to the donor gNB 200.

The RRC message and/or F1 message sent to the donor gNB 200 may includea message notifying a measured load state (particularly a state ofresource load). Such a message may be referred to as a Resource StatusUpdate message. For example, each of the IAB node 300-1 and the IAB node300-2 measures a usage rate of time/frequency resources, which are radioresources, and a usage rate of hardware resources (CPU, memory, and thelike), and sends a message including a measurement result to the donorgNB 200. The usage rate of time/frequency resources may be a usage rateof time/frequency resources in the access link or a usage rate oftime/frequency resources in the backhaul link.

The RRC message and/or F1 message sent to the donor gNB 200 may includea message notifying a measured communication delay state. For example,each of the IAB node 300-1 and the IAB node 300-2 measures at least oneof backhaul delay time, hop count (for example, hop count to the donorgNB 200), and scheduling delay, and sends a message including ameasurement result to the donor gNB 200. Here, measurement of delay time(backhaul delay, scheduling delay) is time until a response totransmission of a message is received, and obtained by measuring timeuntil a response is returned after performing, for example, poling byRLC. Further, the hop count is measured (for each bearer) by counting upthe number in the header each time the Adaptation layer or the likerelays data and reading this value. Alternatively, each IAB node maynotify its own hop count and the hop count to the UE may be calculatedbased on the notification.

Note that these messages may be defined as a response to a request(inquiry) from the gNB 200. Alternatively, these messages may beperiodically sent to the donor gNB 200 or may be sent to the donor gNB200 by an event trigger. In the case of an event trigger, the donor gNB200 may configure a threshold value for the event trigger (radio statethreshold value, load state threshold value, delay state thresholdvalue) for the IAB node 300 and the UE 100 under the donor gNB 200.

Further, each of a radio state, a load state, and a communication delaystate may be measured and notified of separately for the uplink (UL) andthe downlink (DL), or a result (for example, an average value) obtainedby statistically processing a UL measurement result and a DL measurementresult may be notified of.

Furthermore, one RRC message or one F1 message that includes two or moreof a radio state, a load state, and a communication delay state may bedefined. The donor gNB 200 manages or changes the connectionrelationship (topology) of the IAB node 300 and the UE 100 under thedonor gNB 200, and manages or changes a data transfer route (routingtable) based on the RRC message and/or the F1 message. For example, thedonor gNB 200 changes the connection relationship and/or the datatransfer route by performing handover of the IAB node 300 under thedonor gNB 200. The handover of the IAB node 300 will be described in afourth embodiment.

FIG. 12A illustrates an example of creation of a connection relationship(topology) in the donor gNB 200, and FIG. 12B illustrates an example ofcreation of a routing table in the donor gNB 200. In FIGS. 12A and 12 B,the donor gNB 200 is denoted as “D”, six of the IAB nodes 300 aredenoted as “1” to “6”, and the UE (terminal) 100 is denoted as “U”.

As illustrated in FIG. 12A, the donor gNB “D” derives a relationshipbetween nodes for which a data transfer route (route) is likely to beestablished based on a radio state (RSRP, and the like) notified fromthe IAB node 300 and the UE 100 under the donor gNB “D”. For example,the donor gNB “D” determines that a route can be established betweennodes in a case where a radio state between the nodes satisfies certainquality.

In the example of FIG. 12A, the donor gNB “D” determines that fourroutes can be configured between the donor gNB “D” and the UE “U”. Thefirst route is “D”→“1”→“2”→“4”→“U”. The second route is“D”→“1”→“2”→“5”→“U”. The third route is “D”→“3”→“5”→“U”. The fourthroute is “D”→“3”→“6”→“U”. The donor gNB “D” may notify the correspondingIAB nodes of information on these routes and set the informationthereto. For such notification and setting, for example, an RRCreconfiguration message may be used. Note that, although routeconfiguration in a downlink is assumed here, route configuration in anuplink may be performed.

As illustrated in FIG. 12B, the donor gNB “D” determines a load stateand a communication delay state between nodes based on a load state anda communication delay state notified from the IAB node 300 under thedonor gNB “D”, and selects and sets one of the four routes.

In the example of FIG. 12B, in each of the first route(“D”-“1”-“2”-“4”-“U”) and the second route (“D”-“1”-“2”-“5”-“U”), thehop count indicating the number of IAB nodes on the route is three(three hops). In contrast, in each of the third route (“D”-“3”-“5”-“U”)and the fourth route (“D”-“3”-“6”-“U”), the hop count indicating thenumber of IAB nodes on the route is two (two hops). In such a case, thedonor gNB “D” may preferentially select the first route and the secondroute.

Further, in the example of FIG. 12B, the backhaul link between the IABnode “2” and the IAB node “4” and the backhaul link between the IAB node“2” and the IAB node “5” are in a high load state (Loaded), and theaccess link between the IAB node “6” and the UE “U” is in a high loadstate (Loaded). In such a case, the donor gNB “D” may preferentiallyselect the third route that does not go through a link in a high loadstate.

When selecting a route, the donor gNB “D” updates a routing tableaccording to the selected route and sends a message (for example, an RRCreconfiguration message) for configuring the selected route to the nodeon the route. After that, the donor gNB “D” updates the routing table atany time according to the load state and the communication delay stateof each link.

[Variation of the Third Embodiment]

In the third embodiment described above, each of the IAB nodes 300 underthe donor gNB 200 may notify the donor gNB 200 of MBMS-relatedinformation regarding interest in a Multimedia Broadcast MulticastService (MBMS) service. The MBMS-related information may be an RRCmessage or an F1 message.

The MBMS-related information transmitted by the IAB node 300 may berelay necessity information of the MBMS service in the IAB node 300. TheMBMS-related information transmitted by the IAB node 300 may beinformation indicating the number of apparatuses interested in the MBMSservice among lower apparatuses (the UE 100 and the IAB node 300) underthe IAB node 300. The MBMS-related information may be provided for eachidentifier (for example, Temporary Mobile Group Identity (TMGI)) of theMBMS service.

In the present variation, the donor gNB 200 manages or modifies at leastone of the connection relationship (topology) and the data transferroute based on the MBMS-related information notified from each of theIAB nodes 300 under the donor gNB 200. For example, the donor gNB 200manages or changes the data transfer route for the MBMS service based onthe notified MBMS-related information.

In the present variation, each of the IAB nodes 300 may belong to atleast one MBMS Service Area. The MBMS Service Area is a unit of an areain which the same MBMS service is provided. In such a case, the donorgNB 200 may configure the MBMS Service Area to the IAB node 300 whenconfiguring the F1 interface with the IAB node 300. For example, the F1Setup Request message and the F2 Setup Response message may include anMBMS Service Area Identity List.

Fourth Embodiment

The fourth embodiment will be described based mainly on a differencefrom the first to third embodiments described above. The fourthembodiment may be implemented in combination with at least one of thefirst to third embodiments described above.

A communication control method according to the fourth embodiment is amethod in a mobile communication system in which a data transfer routevia at least one IAB node 300 is configured between the donor gNB 200and the UE 100. In the fourth embodiment, the upper apparatus transmitsa handover request for performing handover of the IAB node 300 under theupper apparatus to another upper apparatus. Here, the upper apparatus isanother IAB node (upper IAB node) under the donor gNB 200 or the donorgNB 200. The upper apparatus may be an apparatus that has an RRCconnection with the IAB node 300 for which handover needs to beperformed (that is, an apparatus that has an RRC layer). In the fourthembodiment, an example in which an upper apparatus as a transmissionsource of a handover request is the donor gNB 200 will be mainlydescribed. Further, another upper apparatus as a transmissiondestination of a handover request is a gNB different from the donor gNB200 which is a transmission source of a handover request, or another IABnode under the gNB. In the fourth embodiment, an example in whichanother upper apparatus as a transmission source of a handover requestis a gNB will be mainly described. Specifically, the handover request istransmitted and received on the Xn interface, which is an interfacebetween base stations.

In the fourth embodiment, the handover request includes informationindicating whether or not the IAB node 300 for which handover needs tobe performed is in a relaying state. The state in which the IAB node 300performs data relay may be a state in which at least one of the UE 100exists under the IAB node 300. Specifically, in a case where the relayfunction is activated in the IAB node 300 for which handover needs to beperformed, the IAB node 300 establishes a backhaul link, and there is atleast one of the UE 100 under the IAB node 300, the IAB node 300 can beregarded as being in a state of performing data relay. That there is atleast one of the UE 100 under the IAB node 300 includes not only thatthe UE 100 is connected by radio to the IAB node 300, but also that theUE 100 is connected to the IAB node 300 via at least one another IABnode. Alternatively, the state in which the IAB node 300 performs datarelay may mean a state in which a data transfer route via the IAB node300 is configured. Hereinafter, the state in which the IAB node 300performs data relay is referred to as “IAB active state”.

In contrast, the state in which the IAB node 300 does not perform datarelay may be a state in which at least one of the UE 100 does not existunder the IAB node 300. Specifically, in a case where the relay functionis not activated in the IAB node 300 for which handover needs to beperformed, or the IAB node 300 does not establish a backhaul link, theIAB node 300 can be regarded as being in a state of not performing datarelay. Alternatively, in a case where the IAB node 300 establishes abackhaul link and there is no UE 100 under the IAB node 300, the IABnode 300 can be regarded as being in a state of not performing datarelay. Alternatively, the state in which the IAB node 300 does notperform data relay may mean a state in which a data transfer route viathe IAB node 300 is not configured. Hereinafter, the state in which theIAB node 300 does not perform data relay is referred to as “IAB idlestate”.

As described above, a source gNB notifies a target gNB of whether theIAB node 300 for which handover needs to be performed is in the IABactive state or the IAB idle state at the time of the handover request.In this manner, the target gNB can predict how much the load of thetarget gNB will increase by the handover, so that it is possible toappropriately determine whether or not to accept the handover request.Note that, in a case where handover is performed for the IAB node 300under an upper IAB node from the upper IAB node to another IAB node, thesource IAB node may notify a handover request of the target IAB node inthe same way as that the source gNB notifies the target gNB of ahandover request. In the description below, the source gNB and thetarget gNB may be replaced with the source IAB node and the target IABnode, respectively.

First, an example of handover of the IAB node 300 in the IAB idle statewill be described with reference to Steps S208 to S211 of FIG. 8.

As illustrated in FIG. 8, in Step S208, the gNB 200-1 (source gNB)transmits a handover request message to a gNB 200-2 (target gNB) on theXn interface. Here, the gNB 200-1 (source gNB) includes, in the handoverrequest message, information indicating that the IAB node 300-1 forwhich handover needs to be performed is in the IAB idle state. Like thefirst embodiment, the gNB 200-1 (source gNB) may include, in thehandover request, the IAB indication received from the IAB node 300-1.Alternatively, the gNB 200-1 may send, instead of including the IABindication in the handover request message, information indicating arequest that the target gNB functions as the donor gNB of the IAB node300-1.

The gNB 200-2 determines whether or not to accept the handover of theIAB node 300-1 in consideration of the information included in thehandover request message and indicating that the IAB node 300-1 is inthe IAB idle state. Here, the description will proceed on the assumptionthat the gNB 200-2 determines to accept the handover of the IAB node300-1.

In Step S209, the gNB 200-2 sends a handover acknowledgment message tothe gNB 200-1 on the Xn interface.

In Step S210, the gNB 200-1 transmits a handover instruction message(RRC reconfiguration message) to the IAB node 300-1 based on thehandover acknowledgment message from the gNB 200-2. The handoverinstruction message includes information that designates (a cell of) thegNB 200-2 as a handover destination.

In Step S211, the IAB node 300-1 performs handover for the gNB 200-2based on the handover instruction message from the gNB 200.

Next, an example of handover of the IAB node 300 in the IAB active statewill be described with reference to FIG. 13. FIG. 13 is a diagramillustrating an example of handover of the IAB node 300 in the IABactive state.

As illustrated in FIG. 13, the IAB node (IAB node #1) 300-1 is connectedby radio to the donor gNB (IAB donor #1) 200-1, and the IAB node (IABnode #2) 300-2 and the UE (UE #1) 100-1 are connected by radio to theIAB node 300-1. Further, UEs (UEs #2 to #4) 100-2 to 100-4 are connectedby radio to the IAB node 300-2. Each of the IAB nodes 300 and each ofthe UEs 100 has an RRC layer.

In the example of FIG. 13, it is assumed that handover of the IAB node300-1 is performed from the donor gNB 200-1 to the donor gNB 200-2. TheIAB node 300-1 is in the IAB active state because there are four of theUEs 100-1 to 100-4 and the IAB node 300-2 under the IAB node 300-1.

The donor gNB 200-1 (source gNB) sends a handover request message to thedonor gNB 200-2 (target gNB) on the Xn interface. Here, the donor gNB200-1 (source gNB) includes, in the handover request message,information indicating that the IAB node 300-1 for which handover needsto be performed is in the IAB active state.

As illustrated in FIG. 13, in a case where handover of the IAB node300-1 in the IAB active state is performed from the donor gNB 200-1 tothe donor gNB 200-2, handover is performed collectively for the IAB node300-2 and the UEs 100-1 to 100-4 under the IAB node 300-1. However, itis only required that IAB node 300-1 performs a random access procedurefor the donor gNB 200-2, and the IAB node 300-2 and the UEs 100-1 to100-4 under the IAB node 300-1 do not need to perform a random accessprocedure for the donor gNB 200-2.

Further, the donor gNB 200-1 collectively transfers context informationof the IAB nodes 300 (300-1 and 300-2) under the donor gNB 200-1 and theUEs 100 (100-1 to 100-4) under the donor gNB 200-1 to the donor gNB200-2. The donor gNB 200-1 may include these pieces of contextinformation in one handover request and transmit the handover request tothe donor gNB 200-2.

In a case where handover of the IAB node 300-1 in the IAB active stateis performed from the donor gNB 200-1 to the donor gNB 200-2, the donorgNB 200-1 may transmit a handover request including load informationrelated to the IAB nodes 300 (300-1 and 300-2) under the donor gNB 200-1and/or the UEs 100 (100-1 to 100-4) under the donor gNB 200-1 to thedonor gNB 200-2.

Here, the load information may include the number of UEs 100 (100-1 to100-4) under the donor gNB 200-1. For example, the donor gNB 200-1identifies the number of the UEs 100 (100-1 to 100-4) under the donorgNB 200-1 based on the number of UE contexts managed by the donor gNB200-1 itself or the number of Cell-Radio Network Temporary Identifiers(C-RNTIs), or the routing information that the donor gNB 200-1 managesitself, and includes the identified number in the handover request.However, in a case where a plurality of data transfer routes areconfigured for one UE 100 and the plurality of data transfer routesinclude a primary route and a secondary route, the UE 100 that does nothave a primary route with the donor gNB 200-1 may be excluded. In such acase, the donor gNB 200-1 includes, in the handover request, as loadinformation, the number of the UEs 100 that have a primary route withthe donor gNB 200-1 among the UEs 100 under the donor gNB 200-1.Alternatively, the number of bearers used by the UE 100 may be includedin the handover request as load information instead of the number of theUEs 100. The QoS information associated with the bearer may be furtherincluded in the handover request. The load information may include thenumber of the IAB nodes 300 (300-1 and 300-2) under the donor gNB 200-1.The load information may include the number of layers (hops) of the IABnodes 300 under the donor gNB 200-1 and/or the number of the IAB nodesand/or the UEs 100 under each layer.

Furthermore, the donor gNB 200-1 may include, in the handover request,the MBMS-related information described in the variation of the thirdembodiment.

Fifth Embodiment

The fifth embodiment will be described based mainly on a difference fromthe first to fourth embodiments described above. The fifth embodimentmay be implemented in combination with at least one of the first tofourth embodiments described above.

In the fourth embodiment, the example in which the donor gNB 200 takesthe lead in performing the handover of the IAB node 300 is described. Incontrast, the fifth embodiment is an embodiment in which the IAB node300 takes the lead in performing RRC Re-establishment. The IAB node 300performs RRC Re-establishment when a failure occurs in the radioconnection with an upper apparatus to which the IAB node 300 isconnected. Such a failure in the radio connection is sometimes referredto as Radio Link Failure (RLF).

FIG. 14 is a diagram illustrating an example of an operation environmentaccording to the fifth embodiment. FIG. 14 has the same basicconfiguration as FIG. 13, but differs from FIG. 13 in that an IAB node(IAB node #3) 300-3 is connected by radio to the donor gNB 200-2. Undersuch an environment, it is assumed that the IAB node 300-2 under thedonor gNB 200-1 detects RLF with the IAB node 300-1.

When detecting RLF with the IAB node 300-1, the IAB node 300-2 searchesfor the gNB 200 or the IAB node 300 that can be connected to, other thanthe IAB node 300-1, and attempts to reestablish a radio connection tothe found gNB 200 or IAB node 300. In the example of FIG. 14, the IABnode 300-2 finds the IAB node 300-3 under the donor gNB 200-2, which isdifferent from the donor gNB 200-1 before RLF, and attempts toreestablish a radio connection to the IAB node 300-3.

However, when the IAB node 300-2 is connected to the IAB node 300-3under another donor gNB 200-2, the UE context of the UEs 100-2 to 100-4under the IAB node 300-2 needs to be transferred from the donor gNB200-1 to the donor gNB 200-2. Further, it may also be necessary toestablish a new F1 interface between the donor gNB 200-2 and the IABnode 300-2. Therefore, the above is not preferable from the viewpoint ofa signaling amount and delay. In contrast, when, after detecting RLFwith the IAB node 300-1, the IAB node 300-2 is connected to the donorgNB 200-1 or connected to another IAB node (not illustrated) under thedonor gNB 200-1, increase in a signaling amount and delay can besuppressed.

Note that, instead of the case where RLF occurs between the IAB node300-2 and the IAB node 300-1 as described above, in a case where RLFoccurs between the donor gNB 200-1 and the IAB node 300-1 and the IABnode 300-1 reestablishes a radio connection with the donor gNB 200-2 orthe IAB node 300-3, a similar problem occurs.

In the fifth embodiment, the IAB node 300 having a radio connection tothe donor gNB 200 or an IAB node under the donor gNB 200 acquires andstores a first identifier associated with the donor gNB 200. When, forexample, detecting RLF, the IAB node 300 acquires, from a candidateapparatus that is a candidate for a reestablishment destination of aradio connection, a second identifier associated with the donor gNBcorresponding to the candidate apparatus. The IAB node 300 reestablishesa radio connection for the candidate apparatus in a case where the firstand second identifiers are associated with the same donor gNB 200.

Such an identifier may be referred to as “IAB Area ID” or “IAB topologyID”. A unique identifier is assigned to a group consisting of one donorgNB 200 and the IAB node 300 under the donor gNB 200. The uniqueidentifier may be the ID of the donor gNB 200 (gNB ID) or the cell ID ofthe donor gNB 200 (for example, CGI: Cell Global Identity). Such anidentifier is configured by the donor gNB 200 to the IAB node 300 by,for example, an RRC Reconfiguration message at the time of set up of theIAB node, at the time of configuration change, or the like. Each IABnode 300 broadcasts an identifier configured for the IAB node 300 itselfby SIB or the like. Note that the gNB 200 also broadcasts the aboveidentifier by SIB or the like.

In the example of FIG. 14, the IAB node 300-2 acquires and stores thefirst identifier associated with the donor gNB 200-1. The IAB node 300-2may use the identifier configured by the RRC Reconfiguration message bythe donor gNB 200-1 as the first identifier, or the identifier broadcastby the IAB node 300-1 as the first identifier.

Then, when detecting RLF with the IAB node 300-1, the IAB node 300-2searches for a candidate apparatus (candidate cell) that is a candidatefor a reestablishment destination of a radio connection. In thesearching, the IAB node 300-2 searches for a connectable cell by a cellsearch, and, in a case where a receiving state of a reference signalfrom a cell found by the search is better than a threshold value, theIAB node 300-2 may regard the cell as a candidate apparatus (candidatecell).

The IAB node 300-2 acquires, as the second identifier, an identifier(IAB Area ID) broadcast by the found candidate apparatus. The IAB node300-2 compares the stored first identifier with the acquired secondidentifier. In a case where these identifiers are the same, it meansthat the found candidate apparatus is under the same donor gNB 200 asthat before RLF. Therefore, the IAB node 300-2 attempts to reestablish aradio connection to the found candidate apparatus in a case where theseidentifiers are the same.

Note that the IAB node 300-2 generally selects a candidate apparatushaving a most excellent receiving state based on the rankings in a casewhere a plurality of candidate apparatuses whose reference signals arein a receiving state that satisfies the threshold value are found as aresult of the cell search. Therefore, the IAB node 300-2 may select acandidate apparatus (cell) that broadcasts the same identifier as thestored identifier without depending on the rankings by giving a highestpriority to the candidate apparatus (cell). Alternatively, the IAB node300-2 may select the candidate apparatus (cell) by providing an infiniteoffset to a receiving state of a candidate apparatus (cell) thatbroadcasts the same identifier as the stored identifier and setting thecandidate apparatus (cell) at a highest rank at the time of ranking.

However, the IAB node 300-2 may be unable to find a candidate apparatus(cell) that broadcasts the same identifier as the stored identifier as aresult of the cell search. In such a case, the IAB node 300-2 will beconnected to the donor gNB 200 different from that before RLF or the IABnode under the donor gNB 200. In a case where the IAB node 300-2 isconnected to the donor gNB 200 different from that before RLF or the IABnode under the donor gNB 200, the stored identifier (first identifier)may be notified to the apparatus (cell) as a connection destination. Theapparatus as a connection destination may give notification to the donorgNB 200 corresponding to the identifier notified from the IAB node300-2, so that the original donor gNB 200 updates topology managementand routing management of its own network. Alternatively, the apparatusas a connection destination may request the donor gNB 200 correspondingto the identifier notified from the IAB node 300-2 to transfer contextinformation corresponding to the IAB node 300-2. Alternatively, theapparatus as a connection destination may perform handover of the IABnode 300-2 for the donor gNB 200 corresponding to the identifiernotified from the IAB node 300-2.

Note that, in the present embodiment, the operation in which the IABnode 300 selects a reestablishment destination of a radio connection ismainly described. However, the UE 100 may perform such an operation.

[Variation 1 of the Fifth Embodiment]

In the fifth embodiment described above, the IAB node 300-2 may acquire,from the donor gNB 200-1, and store a list of identifiers (gNB ID andDGI) of each of the donor gNB 200-1 and the IAB node 300 under the donorgNB 200-1 before RLF is detected. The donor gNB 200-1 may provide anupdated list to the IAB node 300-2 each time the IAB node 300 under thedonor gNB 200-1 is added or deleted. In detection of RLF, the IAB node300-2 may search for an apparatus (cell) having an identifier in thestored list and attempt to reestablish a radio connection to the foundapparatus (cell).

[Variation 2 of the fifth embodiment] The variation 2 of the fifthembodiment relates to the details of the variation 1 of the fifthembodiment. The method according to the present variation is a methodexecuted by the IAB node 300 connected to an upper apparatus and havinga function of relaying communication between the upper apparatus and alower apparatus.

Here, the upper apparatus is another IAB node (upper IAB node) under thedonor gNB 200 or the donor gNB 200. The lower apparatus is another IABnode (lower IAB node) under the IAB node 300 or the UE 100.

FIG. 15 illustrates a method according to the variation 2 of the fifthembodiment. As illustrated in FIG. 15, the method according to thepresent variation comprises step S501 for receiving, from the upperapparatus, priority information for establishing the priority of thecandidate upper apparatuses that are candidates for the reconnectiondestination of the IAB node 300, step S503 for detecting a connectionfailure with the upper apparatus after starting the relay (S502), stepS504 for determining an upper apparatus to be the reconnectiondestination of the IAB node 300 from the candidate upper apparatusesbased on the priority information in response to the detection of theconnection failure, and step S505 for transmitting a reconnectionrequest to the determined upper apparatus.

Prior to the flow of FIG. 15, the upper apparatus (for example, thedonor gNB 200) generates priority information based on the routinginformation or the like managed by itself. For example, the upperapparatus includes the cell identifier and/or node identifier of eachIAB node under the same donor gNB 200 in the priority information. Theupper apparatus may include the cell identifier and/or node identifierof the donor gNB 200 in the priority information. The upper apparatusmay give a high priority to the IAB node having a small hop count to thedonor gNB 200, or may give a high priority to the IAB node having asmall traffic or the IAB node having a large communication capacity onthe route to and from the donor gNB 200.

The priority information may include a list of cell identifiers or alist of node identifiers. These identifiers may be arranged indescending order or ascending order of priority. The priorityinformation may include a priority identifier indicating the priorityassociated with each identifier. The priority identifier may be a1-value (High, etc.), a 2-value (Low/High, etc.), and/or a numericalvalue (0 to 7, etc.). The priority information may include an offsetvalue associated with each identifier. The offset value may be an offset(for example, an offset of dB) with respect to a radio measurementresult such as a reference signal received power.

The upper apparatus transmits priority information to the IAB node 300,for example, during the initial setup of the IAB node 300 (see the firstembodiment). The upper apparatus may transmit an RRC message (such asRRC Reconfiguration) including priority information to the IAB node 300,or may transmit an F1 message (such as F1 Setup) including priorityinformation to the IAB node 300. Note that, the F1 message is a messagesent and received on the F1 interface.

The details of each step in FIG. 15 will be hereinafter described.

Step S501:

The IAB node 300 receives, from the upper apparatus, the priorityinformation establishing the priority of the candidate upper apparatusesthat are candidates for the reconnection destination of the IAB node300. For example, the IAB node 300 receives, via another IAB node (upperIAB node), the priority information transmitted from the donor gNB 200.The IAB node 300 stores the received priority information.

Step S502:

The IAB node 300 starts an operation of relaying communication betweenthe upper apparatus and the lower apparatus. In other words, the IABnode 300 activates the relay function. At this time, the IAB node 300 isin the RRC connected mode.

Step S503:

The IAB node 300 detects a connection failure with the upper apparatus.Specifically, the IAB node 300 detects a radio link failure (RLF) of abackhaul link.

Step S504:

Based on the priority information received in step S501, the IAB node300 determines the upper apparatus to be the reconnection destination ofitself (IAB node 300) from the candidate upper apparatuses.Specifically, the IAB node 300 preferentially selects a cell whosepriority information includes a cell identifier and/or another IAB nodewhose priority information includes a node identifier.

The IAB node 300 considers priority information in cell reselectionafter RLF occurs.

In general cell reselection, the UE 100 measures the radio quality(reference signal received power, etc.) of each adjacent cell andselects a cell having a high frequency priority and a high rank as anappropriate cell by ranking based on the frequency priority and radiomeasurement result from the cells satisfying a predetermined radioquality standard (S-criterion).

On the other hand, in cell reselection in consideration of the priorityinformation, the IAB node 300 may select, as an appropriate cell, thecell having the highest priority in the priority information from thecells satisfying the predetermined radio quality standard (S-criterion).

On the other hand, in cell reselection in consideration of the priorityinformation, the IAB node 300 may perform ranking targeting only cellswhose priority information includes a cell identifier from the cellssatisfying the predetermined radio quality standard (S-criterion), andselect the cell with the highest rank as an appropriate cell. Here, in acase where the priority information includes an offset value, the IABnode 300 may correct the rank by adding the offset value to the radiomeasurement result.

Alternatively, in the cell reselection in consideration of the priorityinformation, the IAB node 300 may perform ranking based on the radiomeasurement result from the cells satisfying the predetermined radioquality standard (S-criterion), and select, as an appropriate cell, thecell whose rank is high and whose priority information includes the cellidentifier.

Here, an example in which the IAB node 300 considers priorityinformation in cell reselection after RLF occurs has been described.However, the IAB node 300 may consider priority information at the timeof a random access procedure (specifically, transmission of areconnection request) after cell reselection.

Note that the priority information may be applied only after the RLFoccurs, or may be applied only when operating as an IAB node (after theIAB node specific configuration has been made).

Step S505:

The IAB node 300 transmits a reconnection request to the upper apparatus(appropriate cell) determined in step S504.

Specifically, the IAB node 300 performs a random access procedure forthe upper apparatus (appropriate cell) determined in step S504. In therandom access procedure, the IAB node 300 transmits a random accesspreamble to the upper apparatus (appropriate cell) determined in stepS504, and receives a random access response from the upper apparatus.

The IAB node 300 transmits a reconnection request to the upper apparatusin response to reception of the random access response. The reconnectionrequest may be an RRC Re-establishment Request message, which is an RRCmessage.

Note that the RRC Re-establishment procedure is successful in a casewhere the context information of the IAB node 300 is available on thenetwork side, and an acknowledgment (for example, an RRCRe-establishment message) is transmitted from the upper apparatus of thereconnection destination to the IAB node 300. On the other hand, in acase where the context information of the IAB node 300 is not availableon the network side, the RRC Re-establishment procedure fails, and anegative acknowledgment (for example, RRC Re-establishment Rejectmessage) is transmitted from the upper apparatus of the reconnectiondestination to the IAB node 300.

Here, as described above, the context information is informationincluding AS layer connection configuration on the radio side (contentof RRC reconfiguration), PDU session resource configuration on thenetwork side (such as UE ID and session ID, QoS/slice configuration ofAMF or RAN), and other related information (such as history informationand preference information on behavior, communications, and the like ofthe IAB node.

For example, in a case where the IAB node 300 determines the cell ofanother IAB node under the same donor gNB 200 as the reconnectiondestination, the context information is held in the donor gNB 200 andthe context information is available, and hence RRC Re-establishmentprocedure is considered successful. When the RRC Re-establishmentprocedure is successful, the IAB node 300 can change the connectiondestination while maintaining the RRC connected mode.

On the other hand, in a case where the IAB node 300 determines the cellof another IAB node under the same donor gNB 200 as the reconnectiondestination, or in a case where the backhaul link of the other IAB nodehas a failure or the other IAB node has a congestion, the RRCRe-establishment procedure can fail. That is, the IAB node 300 receivesan RRC Re-establishment Reject message. Here, the RRC Re-establishmentReject message may include information indicating the cause of thereconnection reject (for example, “no connection to IAB topology”). Insuch a case, the IAB node 300 may select the cell having the secondhighest priority (or rank) and transmit a reconnection request to theselected cell.

In the present variation, an example in which the priority informationincludes a list of cell identifiers and/or a list of node identifiershas been described. The priority information may include a list offrequencies. Each frequency may be expressed by an identifier indicatinga carrier frequency, or may be expressed by the center frequency andbandwidth of the carrier frequency. Each frequency may be expressed by afrequency channel identifier within the carrier frequency or a resourceblock identifier. In a case where the priority information includes alist of frequencies, the priority information may include a priorityidentifier for each frequency or may include an offset value for anexisting frequency priority.

[Variation 3 of the Fifth Embodiment]

The variation 3 of the fifth embodiment will be described based mainlyon a difference from the variation 2 of the fifth embodiment. Thevariation 3 of the fifth embodiment may be used in combination with thefifth embodiment or its variation described above.

The method according to the present variation is a method executed bythe IAB node 300 connected to an upper apparatus and having a functionof relaying communication between the upper apparatus and a lowerapparatus.

FIG. 16 illustrates a method according to the variation 3 of the fifthembodiment. As illustrated in FIG. 16, the method according to thepresent variation comprises step S512 for detecting a connection failurewith the upper apparatus after starting the relay (S511), step S513 forreceiving the system information broadcasted from the candidate upperapparatus to be the reconnection destination of the IAB node 300, stepS514 for determining an upper apparatus to be the reconnectiondestination of the IAB node 300 from the candidate upper apparatusesbased on the system information in response to the detection of theconnection failure, and step S515 for transmitting a reconnectionrequest to the determined upper apparatus. The system informationincludes at least one of information indicating beingpossible/impossible to accept the IAB node 300 in the candidate upperapparatus and information regarding the load amount (that is, the upperlimit value of the load amount) that the candidate upper apparatus canaccept. The load amount that can be accepted by the candidate upperapparatus may be read as the resource amount that the candidate upperapparatus can provide.

The details of each step in FIG. 16 will be hereinafter described.

Step S511:

The IAB node 300 starts an operation of relaying communication betweenthe upper apparatus and the lower apparatus. In other words, the IABnode 300 activates the relay function. At this time, the IAB node 300 isin the RRC connected mode.

Step S512:

The IAB node 300 detects a connection failure with the upper apparatus.Specifically, the IAB node 300 detects a radio link failure (RLF) of abackhaul link.

Step S513:

The IAB node 300 receives the system information (System InformationBlock: SIB) broadcasted from the candidate upper apparatus to be thereconnection destination.

The SIB may include information indicating being possible/impossible toaccept the IAB node 300 in the candidate upper apparatus, for example,“no connection to IAB topology” indicating being impossible to accept,or “connection available to IAB topology” indicating being possible toaccept. In a case where the congestion level of the candidate upperapparatus is low, the candidate upper apparatus may broadcast, by theSIB, information indicating that the IAB node 300 is acceptable. On theother hand, in a case where the congestion level of the candidate upperapparatus is high, the candidate upper apparatus may not broadcast, bythe SIB, information indicating that the IAB node 300 is acceptable.

The SIB may include information regarding the load amount that thecandidate upper apparatus can accept. For example, such information maybe information indicating the number of UEs 100 that the candidate upperapparatus can accept (the number of UEs 100 that are in datacommunication or in the RRC connected mode), information indicating thenumber of bearers that the candidate upper apparatus can accept, orinformation indicating the data amount that the candidate upperapparatus can accept. Based on the congestion level of the candidateupper apparatus, the candidate upper apparatus broadcasts, by the SIB,information on the load amount that the candidate upper apparatus canaccept.

Step S514:

Based on the system information received in step S514, the IAB node 300determines the upper apparatus to be the reconnection destination of theIAB node 300 from the candidate upper apparatuses. The IAB node 300 mayexclude a candidate upper apparatus whose system information does notsatisfy the conditions from the priority information according to thevariation 2 of the fifth embodiment.

For example, the IAB node 300 selects an appropriate cell by performingcell reselection only for the candidate upper apparatus (cell)broadcasting, by the SIB, information indicating that the IAB node 300is acceptable. The method according to the variation 2 of the fifthembodiment can be applied to the cell reselection.

In a case where the IAB node 300 receives the SIB including informationon the acceptable load amount from the candidate upper apparatus, in acase where the IAB node 300 determines that the load amount of itself(IAB node 300) and its lower apparatus (for example, the number of UEs,the number of bearers, and the data amount) are equal to or less thanthe acceptable load amount, the candidate upper apparatus is included inthe cell reselection target (candidate). The method according to thevariation 2 of the fifth embodiment can be applied to the cellreselection.

Step S515:

The IAB node 300 transmits a reconnection request to the upper apparatus(appropriate cell) determined in step S504. Such an operation is thesame as in the variation 2 of the fifth embodiment.

[Variation 4 of the Fifth Embodiment]

The variation 4 of the fifth embodiment will be described based mainlyon a difference from the variations 2 and 3 of the fifth embodiment. Thevariation 4 of the fifth embodiment may be used in combination with thefifth embodiment or its variation described above.

The method according to the present variation is a method executed bythe IAB node 300 connected to an upper apparatus and having a functionof relaying communication between the upper apparatus and a lowerapparatus.

FIG. 17 illustrates a method according to the variation 4 of the fifthembodiment. As illustrated in FIG. 17, the method according to thepresent variation comprises step S522 for detecting a connection failurewith the upper apparatus after starting the relay (step S521), stepsS523 and S524 for initiating a procedure (random access procedure) forperforming reconnection for a candidate upper apparatus that is acandidate for the reconnection destination of the IAB node 300 inresponse to the detection of the connection failure, and step S525 fornotifying the candidate upper apparatus of the information (for example,the number of UEs, the number of bearers, and the data amount) on theload amount of the lower apparatus in the procedure.

The details of each step in FIG. 17 will be hereinafter described.

Step S521:

The IAB node 300 starts an operation of relaying communication betweenthe upper apparatus and the lower apparatus. In other words, the IABnode 300 activates the relay function. At this time, the IAB node 300 isin the RRC connected mode.

Step S522:

The IAB node 300 detects a connection failure with the upper apparatus.Specifically, the IAB node 300 detects a radio link failure (RLF) of abackhaul link.

Step S523:

The IAB node 300 determines the candidate upper apparatus that is acandidate for the reconnection destination. As a determination methodhere, the method of the fifth embodiment or its variation may be used.

Step S524:

The IAB node 300 initiates a random access procedure for the candidateupper apparatus.

Step S525:

The IAB node 300 notifies the candidate upper apparatus of theinformation (for example, the number of UEs, the number of bearers, andthe data amount) on the load amount of the lower apparatus in the randomaccess procedure.

In the random access procedure, the IAB node 300 transmits a randomaccess preamble, receives a random access response from the candidateupper apparatus, and transmits a reconnection request to the upperapparatus in response to the reception of the random access response.The reconnection request may be an RRC Re-establishment Request message,which is an RRC message.

The IAB node 300 may notify information regarding the load amount of thelower apparatus by using a random access preamble. For example, aPhysical Random Access Channel (PRACH) resource, which is a radioresource for random access preamble, is divided into a plurality ofresource regions, and a load amount is associated with each resourceregion. The IAB node 300 selects a resource region corresponding to theload amount of its lower apparatus, and transmits a random accesspreamble using the radio resource in the selected resource region.

Alternatively, the IAB node 300 may notify the information regarding theload amount of the lower apparatus by using a reconnection request (RRCRe-establishment Request message). For example, the informationregarding the load amount of the lower apparatus is included in the RRCRe-establishment Request message and transmitted.

The candidate upper apparatus determines whether or not the IAB node 300is acceptable based on the information regarding the load amountnotified from the IAB node 300. In a case where the candidate upperapparatus determines that the IAB node 300 is acceptable, the candidateupper apparatus transmits an acknowledgment (for example, an RRCRe-establishment message) to the IAB node 300.

On the other hand, in a case where the candidate upper apparatusdetermines that the IAB node 300 is unacceptable, the candidate upperapparatus transmits a negative acknowledgment (for example, RRCRe-establishment Reject message) to the IAB node 300.

[Variation 5 of the Fifth Embodiment]

The variation 5 of the fifth embodiment will be described based mainlyon a difference from the variations 2 to 4 of the fifth embodiment. Thevariation 5 of the fifth embodiment may be used in combination with thefifth embodiment or its variation described above.

The method according to the variation 5 of the fifth embodiment isexecuted by the IAB node 300 connected to an upper apparatus and havinga function of relaying communication between the upper apparatus and alower apparatus.

FIG. 18 illustrates a method according to the variation 5 of the fifthembodiment. As illustrated in FIG. 18, the method according to thepresent variation comprises step S532 for detecting deterioration ofradio quality with the upper apparatus after starting the relay (stepS531), and step S533 for performing predetermined processing for waitingfor recovery of the radio quality with the upper apparatus in responseto the detection of the deterioration of radio quality. Thepredetermined processing includes at least one of (1) processing ofextending the time from detection of the deterioration of radio qualityto determination that a connection failure has occurred, (2) processingof extending the time from the occurrence of a connection failure totransmission of a reconnection request, and (3) processing oftransitioning to the RRC inactive mode.

The details of each step in FIG. 18 will be hereinafter described.

Step S531:

The IAB node 300 starts an operation of relaying communication betweenthe upper apparatus and the lower apparatus. In other words, the IABnode 300 activates the relay function. At this time, the IAB node 300 isin the RRC connected mode.

Step S532:

The IAB node 300 detects deterioration of radio quality with the upperapparatus. The deterioration of radio quality may mean that thesynchronization with the upper apparatus has been lost.

Step S533:

The IAB node 300 performs predetermined processing for waiting forrecovery of the radio quality with the upper apparatus. Thepredetermined processing includes at least one of (1) processing ofextending the time from detection of the deterioration of radio qualityto determination that a connection failure has occurred, (2) processingof extending the time from the occurrence of a connection failure totransmission of a reconnection request, and (3) processing oftransitioning to the RRC inactive mode.

Here, (1) processing of extending the time from detection of thedeterioration of radio quality to determination that a connectionfailure has occurred means extending, from the reference time, thewaiting time for determining that a connection failure (RLF) hasoccurred. The reference time may be the time to be configured for the UE100 or the time used when the IAB node 300 is not performing the relayoperation (the relay function is not activated).

Further, (2) processing of extending the time from the occurrence of aconnection failure to transmission of a reconnection request meansextending, from the reference time, the waiting time for transmitting areconnection request. The reference time may be the time to beconfigured for the UE 100 or the time used when the IAB node 300 is notperforming the relay operation (the relay function is not activated).

Further, (3) processing of transitioning to the RRC inactive mode meanstransitioning from the RRC connected mode to the RRC inactive mode.

Step S534:

The IAB node 300 determines whether or not the radio quality with theupper apparatus has been recovered. The recovery of the radio qualitymay mean that the synchronization with the upper apparatus has beenestablished. The processing proceeds to step S535 if the radio qualitywith the upper apparatus is recovered, and the processing proceeds tostep S536 if the radio quality with the upper apparatus is notrecovered.

Step S535:

The IAB node 300 maintains the connection with the upper apparatus.

Step S536:

The IAB node 300 determines the reconnection destination by the methodof the fifth embodiment or its variation described above, and transmitsa reconnection request to the determined reconnection destination.

[Variation 6 of the Fifth Embodiment]

The variation 6 of the fifth embodiment will be described based mainlyon a difference from the variations 2 to 5 of the fifth embodiment. Thevariation 6 of the fifth embodiment may be used in combination with thefifth embodiment or its variation described above.

The method according to the variation 6 of the fifth embodiment isexecuted by the IAB node 300 connected to an upper apparatus and havinga function of relaying communication between the upper apparatus and alower apparatus.

FIG. 19 illustrates a method according to the variation 6 of the fifthembodiment. As illustrated in FIG. 19, the method according to thepresent variation comprises step S542 for detecting a connection failurewith the upper apparatus after starting the relay (step S541), stepsS543 for transmitting a reconnection request to the first candidateupper apparatus to be the reconnection destination of the IAB node 300,step S544 for receiving a response indicating that the reconnection isimpossible from the first candidate upper apparatus, and step S545 fortransmitting a reconnection request to the second candidate upperapparatus to be the reconnection destination of the IAB node 300 inresponse to reception of the response.

The details of each step in FIG. 19 will be hereinafter described.

Step S541:

The IAB node 300 starts an operation of relaying communication betweenthe upper apparatus and the lower apparatus. In other words, the IABnode 300 activates the relay function. At this time, the IAB node 300 isin the RRC connected mode.

Step S542:

The IAB node 300 detects a connection failure with the upper apparatus.Specifically, the IAB node 300 detects a radio link failure (RLF) of abackhaul link.

Step S543:

The IAB node 300 determines the candidate upper apparatus that is acandidate for the reconnection destination. As a determination methodhere, the method of the fifth embodiment described above or itsvariation may be used. The IAB node 300 determines the second candidateupper apparatus having the second priority in addition to the firstcandidate upper apparatus having the highest priority.

The IAB node 300 transmits a reconnection request to the first candidateupper apparatus. The reconnection request may be an RRC Re-establishmentRequest message. If the IAB node 300 is in the RRC inactive mode, thereconnect request may be an RRC Resume Request message.

Step S544:

The IAB node 300 receives a response indicating that the reconnection isimpossible from the first candidate upper apparatus. In such a case, itis meant that the context information of the IAB node 300 is notavailable by the first candidate upper apparatus.

Here, the response indicating that the reconnection is impossible may bea response message indicating that he reconnection is rejected (forexample, the RRC Re-establishment Reject message), or may be a messageindicating that not the reconnection but an initial connection has to beperformed (for example, an RRC Setup message).

Note that in a general reconnection procedure, upon receiving, from thegNB 200, a response indicating that the reconnection is impossible, theUE 100 makes an initial connection to the gNB 200 (that is, transmits anRRC Request message to the gNB 200). However, the initial connectionrequires configuration to the UE 100 to be redone from the beginning.

Therefore, if the IAB node 300 during the relay operation is caused toexecute such an operation, the adverse effect on the communication ofthe lower apparatus (particularly UE 100) becomes large. Therefore, inthe present variation, upon receiving, from the first candidate upperapparatus, a response indicating that reconnection is impossible, afterstarting the relay, the IAB node 300 does not make an initial connectionto the first candidate upper apparatus but makes an attempt ofreconnection to the second candidate upper apparatus.

Step S545:

The IAB node 300 transmits a reconnection request to the secondcandidate upper apparatus to be the reconnection destination of the IABnode 300. Upon receiving a response indicating that reconnection ispossible from the second candidate upper apparatus, the IAB node 300reconnects to the second candidate upper apparatus.

Here, the IAB node 300 may notify the first candidate upper apparatus ofcancellation of the reconnection request (for example, RRCRe-establishment Cancel, RRC Setup Cancel).

Note that the IAB node 300 may carry out the operation according to thepresent variation only in a case where the operation has been permittedor set in advance by the RRC Reconfiguration or the like from the upperapparatus to the IAB node 300 before the occurrence of RLF.

The operation according to the present variation may be carried out onlyin a case where an attempt timer is in operation. The timer value of thetimer may be set in advance by RRC Reconfiguration or the like. The IABnode 300 may make an initial connection to the first candidate upperapparatus in a case where the reconnection is not successful by the timethe timer expires.

[Variation 7 of the Fifth Embodiment]

The variation 7 of the fifth embodiment will be described based mainlyon a difference from the variations 2 to 6 of the fifth embodiment. Thevariation 7 of the fifth embodiment may be used in combination with thefifth embodiment or its variation described above.

The method according to the variation 7 of the fifth embodiment isexecuted by the IAB node 300 connected to an upper apparatus and havinga function of relaying communication between the upper apparatus and alower apparatus.

FIG. 20 illustrates a method according to the variation 7 of the fifthembodiment. As illustrated in FIG. 20, the method according to thepresent variation comprises step S552 for detecting a connection failurewith the upper apparatus after starting the relay (step S551), stepsS553 for transmitting a reconnection request to the first candidateupper apparatus to be the reconnection destination of the IAB node 300in response to the detection of the connection failure, and steps S553for transmitting a reconnection request to the second candidate upperapparatus to be the reconnection destination of the IAB node 300 beforereceiving the response to the reconnection request from the firstcandidate upper apparatus. The reconnection request may be an RRCRe-establishment Request message.

The details of each step in FIG. 20 will be hereinafter described.

Step S551:

The IAB node 300 starts an operation of relaying communication betweenthe upper apparatus and the lower apparatus. In other words, the IABnode 300 activates the relay function. At this time, the IAB node 300 isin the RRC connected mode.

Step S552:

The IAB node 300 transmits a reconnection request to the first candidateupper apparatus. The IAB node 300 transmits a reconnection request tothe second candidate upper apparatus before receiving the response tothe reconnection request from the first candidate upper apparatus. TheIAB node 300 may concurrently transmit a reconnection request to thefirst candidate upper apparatus and the second candidate upperapparatus.

As a method of determining the candidate upper apparatus, the method ofthe fifth embodiment described above or its variation may be used. Here,the description will proceed on the assumption that the first candidateupper apparatus is higher in priority (rank) than the second candidateupper apparatus.

Upon receiving an acknowledgment (RRC Re-establishment message) fromboth the first candidate upper apparatus and the second candidate upperapparatus, the IAB node 300 may transmit a completion notification (RRCRe-establishment Complete message) to the first candidate upperapparatus having a higher priority, and transmit a reconnectioncancellation notification (RRC Re-establishment Cancel message) to thesecond candidate upper apparatus having a higher priority.

Note that the IAB node 300 may carry out the operation according to thepresent variation only in a case where the operation has been permittedor set in advance by the RRC Reconfiguration or the like from the upperapparatus to the IAB node 300 before the occurrence of RLF.

[Variation 8 of the Fifth Embodiment]

The variation 8 of the fifth embodiment will be described based mainlyon a difference from the variations 2 to 7 of the fifth embodiment. Thevariation 8 of the fifth embodiment may be used in combination with thefifth embodiment or its variation described above.

The method according to the variation 8 of the fifth embodiment isexecuted by the IAB node 300 connected to an upper apparatus and havinga function of relaying communication between the upper apparatus and alower apparatus.

FIG. 21 illustrates a method according to the variation 8 of the fifthembodiment. As illustrated in FIG. 21, the method according to thepresent variation comprises step S562 for detecting a connection failurewith the upper apparatus after starting the relay (step S561), stepsS563 for transmitting a reconnection request to the candidate upperapparatus to be the candidate of reconnection destination of the IABnode 300 in response to the detection of the connection failure, stepS564 for receiving a response indicating that the reconnection isimpossible from the candidate upper apparatus, and step S565 fortransmitting the context information of the IAB node 300 to thecandidate upper apparatus in response to the reception of the response.

The details of each step in FIG. 21 will be hereinafter described.

Step S561:

The IAB node 300 starts an operation of relaying communication betweenthe upper apparatus and the lower apparatus. In other words, the IABnode 300 activates the relay function. At this time, the IAB node 300 isin the RRC connected mode.

Step S562:

The IAB node 300 detects a connection failure with the upper apparatus.Specifically, the IAB node 300 detects a radio link failure (RLF) of abackhaul link.

Step S563:

The IAB node 300 determines the candidate upper apparatus that is acandidate for the reconnection destination. As a determination methodhere, the method of the fifth embodiment described above or itsvariation may be used. The IAB node 300 transmits a reconnection requestto the candidate upper apparatus. The reconnection request may be an RRCRe-establishment Request message. If the IAB node 300 is in the RRCinactive mode, the reconnect request may be an RRC Resume Requestmessage.

Step S564:

The IAB node 300 receives a response indicating that reconnection isimpossible from the candidate upper apparatus. In such a case, it ismeant that the context information of the IAB node 300 is not availableby the candidate upper apparatus.

Here, the response indicating that the reconnection is impossible may bea response message indicating that the reconnection is rejected (forexample, the RRC Re-establishment Reject message), or may be a messageindicating that not the reconnection but an initial connection has to beperformed (for example, an RRC Setup message).

Step S565:

The IAB node 300 transmits the context information of the IAB node 300to the candidate upper apparatus. As described above, the contextinformation includes AS layer connection configuration on the radio side(content of RRC reconfiguration), PDU session resource configuration onthe network side (such as UE ID and session ID, QoS/slice configurationof AMF or RAN), and other related information (such as preferenceinformation and/or history information on behavior, communications, andthe like of the IAB node. By transmitting the context information to thecandidate upper apparatus, the candidate upper apparatus can use thecontext information, so that the reconnection becomes successful.

Step S566:

The IAB node 300 may transmit again the reconnection request (RRCRe-establishment Request message) and transmit its own contextinformation included in the reconnection request or the subsequentadditional message. In a case of using an additional message, the IABnode 300 may include a flag indicating presence of an additional messagein the reconnection request.

Sixth Embodiment

The sixth embodiment will be described based mainly on a difference fromthe first to fifth embodiments described above. The sixth embodiment maybe implemented in combination with at least one of the first to fifthembodiments described above.

The communication control method according to the sixth embodiment is amethod in a mobile communication system in which a data transfer routevia at least one IAB node 300 is configured between the donor gNB 200and the UE 100. In the sixth embodiment, the IAB node 300 sends, to anupper apparatus, a first buffer state report that indicates at least anamount of data available to the IAB node 300 for uplink transmission.Here, the upper apparatus is another IAB node (upper IAB node) under thedonor gNB 200 or the donor gNB 200. The upper apparatus allocates radioresources for uplink transmission to the IAB node 300 based on the firstbuffer state report.

As described above, in the sixth embodiment, a buffer state report (BSR)for the IAB node is introduced, so that an upper apparatus to which theIAB node is connected can appropriately allocate radio resources foruplink transmission to the IAB node.

Specifically, the IAB node 300 has an uplink buffer that temporarilystores data waiting for uplink transmission. For example, the MAC layerof the IAB node 300 notifies the MAC layer of an upper apparatus of afirst buffer state including information indicating a data amount in theuplink buffer. The MAC layer of the upper apparatus has a scheduler,allocates uplink radio resources to the IAB node 300 based on the firstbuffer state, and notifies the IAB node 300 of the allocated resourcesvia the control channel.

Here, since the IAB node 300 buffers uplink data for a plurality of UEs,it is considered that the IAB node 300 has an uplink buffer of largercapacity than the UE 100. Therefore, the buffer state report for the IABnode may have a different format from the buffer state report for theUE. Further, the data amount that can be expressed by the buffer statereport for the IAB node (maximum data amount) may be larger than thedata amount that can be expressed by the buffer state report for the UE(maximum data amount).

The buffer state report for the IAB node may include information on thenumber of UEs 100 under the IAB node 300. The IAB node 300 may determinethe number of UEs 100 under the IAB node 300 based on UE context,C-RNTI, or the like. The donor gNB 200 may notify the IAB node 300 ofthe number of UEs 100 under the IAB node 300. The IAB node 300 mayinclude, in the buffer state report, the number of the UEs 100 that hasdata in the uplink buffer of the IAB node 300 among the UEs 100 underthe IAB node 300. In other words, the IAB node 300 may notify the upperapparatus of how many UEs of uplink data the IAB node 300 has by thebuffer state report. Alternatively, the IAB node 300 may include, in thebuffer state report, the number of the UEs 100 in an RRC connected stateamong the UEs 100 under the IAB node 300.

The buffer state report for the IAB node may take into account not onlythe amount of data actually existing in the uplink buffer of the IABnode 300, but also the buffer state report (that is, a potential uplinkdata amount) from the lower apparatus. In this manner, the upperapparatus can allocate uplink radio resources to the IAB node 300 inadvance in consideration of a potential uplink data amount, so thattransmission delay in an uplink due to multi-hop can be reduced.

The lower apparatus is a lower IAB node and the UE 100 under the IABnode 300. The IAB node 300 receives, from the lower apparatus, a secondbuffer state report indicating the amount of data available to the lowerapparatus for uplink transmission. Based on the second buffer statereport, the IAB node 300 transmits, to the upper apparatus, the firstbuffer state report based on the amount of data available to the IABnode 300 for uplink transmission and the amount of data available to thelower apparatus for uplink transmission. For example, the IAB node 300may include, in the first buffer state report, the sum of the amount ofdata available to the IAB node 300 for uplink transmission and theamount of data available to the lower apparatus for uplink transmission.Alternatively, the IAB node 300 may include, in the first buffer statereport, a first BSR value indicating the amount of data available to theIAB node 300 for uplink transmission and a second BSR value indicatingthe amount of data available to the lower apparatus for uplinktransmission.

Seventh Embodiment

The seventh embodiment will be described based mainly on a differencefrom the first to sixth embodiments described above. The seventhembodiment may be implemented in combination with at least one of thefirst to sixth embodiments described above.

The communication control method according to the seventh embodiment isa method in a mobile communication system in which a data transfer routevia at least one IAB node 300 is configured between the donor gNB 200and the UE 100. In the seventh embodiment, the first IAB node under thedonor gNB 200 or the UE under the first IAB node sends, to the upperapparatus, a request to change a data transfer route from a first routevia the first IAB node to a second route via the second IAB node 300.Here, the upper apparatus is an upper IAB node under the donor gNB 200or the donor gNB 200. In the seventh embodiment, an example in which theupper node is the donor gNB 200 will be mainly described.

FIG. 22 is a diagram illustrating an operation example according to theseventh embodiment. In FIG. 22, the donor gNB 200 is denoted as “D”, twoof the IAB nodes 300 are shown as “1” and “2”, and the UE 100 is denotedas “U”.

As illustrated in FIG. 22, the IAB node 300-1 or the UE 100 sends, tothe donor gNB 200-1, a request to change a data transfer route from thefirst route via the IAB node 300-1 to the second route via the IAB node300-2. Such a request may be an F1 message sent and received on the F1interface, an Xn message sent and received on the Xn interface, or anRRC message. For example, the IAB node 300-1 or the UE 100 may transmitsuch a request in a case where quality (radio state and the like) in thefirst route deteriorates.

Here, in addition to the first route, the second route may beestablished in advance. That is, the first route and the second routemay coexist. The first route may be the primary route (main route) andthe second route may be the secondary route (spare route). In that case,in a case where the amount of data to be transmitted to the first route(resources used for data transmission) reaches the upper limit, the datamay be transmitted to the second route. Alternatively, the first routeis released and the second route may be established. That is, the firstroute and the second route do not have to coexist. Alternatively, in acase where the data transmitted to the first route does not reach atransmission destination (Ack cannot be received from the transmissiondestination), the same data may be transmitted to the second route.

The request sent from the IAB node 300-1 or the UE 100 to the donor gNB200 includes at least one of a request to pass control of the primaryroute to the second IAB node 300, an establishment request requestingestablishment of the second route, a release request requesting releaseof the first route, a change request requesting a configuration changefor the second route that has already been established, and anallocation request requesting resource allocation for the second routethat has already been established. Each of these requests may be definedas a separate message, or one message that combines two or more requestsmay be defined.

Such a message includes, as an information element, at least one of atransmission source identifier of the message, a transmissiondestination identifier of the message, an identifier of a lowerapparatus (the IAB node or the UE that is a child node) for which routechange is to be performed, an identifier of the bearer corresponding tothe first route and/or the second route, and cause information (Cause).In the case of configuration change of the second route or resourceallocation change, the message may further include the resource amountdesired to be changed or the ratio of the data amount flowing to itself(for example, the reduction rate of the data flowing to itself).

For example, the IAB node 300-1 or the UE 100 may request control of theprimary route of the primary route to be passed from the IAB node 300-1to another IAB node after the first route, which is the primary route,is established. The donor gNB 200 identifies another route (the secondroute) in response to the request, and then performs processing ofproviding control to the IAB node 300-2 that constitutes the anotherroute.

Another embodiment

In the above-described embodiments, the processing executed by the IABnode 300 may be regarded as being executed by the function of the userequipment in the IAB node 300. That is, the processing executed by theIAB node 300 may be regarded as being executed by the UE 100. Theprocessing executed by the IAB node 300 may be processing executed bythe UE 100.

An example in which the mobile communication system 1 is a 5G mobilecommunication system is mainly described in the above embodiment.Alternatively, the base station in the mobile communication system 1 maybe an eNB. Furthermore, the core network in the mobile communicationsystem 1 may be Evolved Packet Core (EPC). Furthermore, the gNB may beconnected to the EPC, the eNB may be connected to the 5GC, and the gNBand the eNB may be connected to each other via an inter-base stationinterface (Xn interface, X2 interface).

A program for causing a computer to execute each processing describedthe above embodiment. In addition, the program may be recorded on acomputer-readable medium. If a computer-readable medium is used, aprogram can be installed in the computer. Here, the computer-readablemedium on which the program is recorded may be a non-transitoryrecording medium. The non-transitory recording medium is notparticularly limited, but may be a recording medium such as a CD-ROM ora DVD-ROM. A chip set including a memory that stores a program forexecuting each processing by the UE 100 and the gNB 200 and a processorthat executes the program stored in the memory may be provided.

Note that not limited to the case where each of the above-describedembodiments is independently implemented, two or more embodiments may becombined and implemented. Further, the flows shown in each figure may becombined as appropriate.

Appendix

1. Introduction

RAN Plenary approved the study on Integrated Access and Backhaul (IAB)for NR. One of challenges in this study is the topology management androute selection in response to support of multi-hop networking. The TRcaptures the requirement for the topology adaptation.

5.2.2 Topology adaptation

Wireless backhaul links are vulnerable to blockage, e.g., due to movingobjects such as vehicles, due to seasonal changes (foliage), or due toinfrastructure changes (new buildings). Such vulnerability also appliesto physically stationary IAB-nodes. Also, traffic variations can createuneven load distribution on wireless backhaul links leading to locallink or node congestion.

Topology adaptation refers to procedures that autonomously reconfigurethe backhaul network under circumstances such as blockage or localcongestion without discontinuing services for UEs.

Requirement: Topology adaptation for physically fixed relays shall besupported to enable robust operation, e.g., mitigate blockage and loadvariation on backhaul links.

In this appendix, the initial consideration of topology/route managementprocedure is discussed.

2. Discussion

2.1. Principle of Topology Adaptation Design

The TR captures a couple of IAB topologies, i.e., Spanning Tree (ST) andDirected Acyclic Graph (DAG), and the link/route redundancy in DAG, asshown in FIG. 23.

The (re-)formation of topology would be performed due to node-levelchanges, e.g., a new IAB node installed in the network through theprocedure of “Integration of IAB-node.” It will happen once in a longterm and may be referred as “topology management”.

On the other hand, the (re-)selection of link/route would be caused bylink-level variations, e.g., blockage due to RLF or congestion, and alsodone after the topology management above. It will happen frequentlyrelatively in a short term and may be referred as “route management”.

The “topology adaptation” could be considered as an autonomous(re-)configuration mechanism for both of the “topology management” andthe “route management”, according the TR. In other words, it's preferredthat the topology adaptation is a unified mechanism to be applied to the(re-)formation of IAB network topology and the (re-)selection of routewithin the topology, that includes the initial deployment phase and thenetwork operation phase from the perspectives of IAB nodes and IABdonor.

Observation 1: A unified approach would be preferred for the topologyadaptation to be performed for topology management (e.g., in initialdeployment phase) and route management (e.g., in network operationphase).

In our understanding, the topology adaptation is seen as a kind ofmobility scenario, so it's basically realized by the existing mobilityfunctions, i.e., handover and cell (re-)selection. It may imply that thetopology adaptation has a couple of design options, i.e., NW-controlledapproach and UE-based approach; or it will be more suitable to sayeither IAB donor-controlled mechanism or IAB node-based mechanism, inthe context of IAB study.

Observation 2: Topology adaptation would have a couple of designoptions, i.e., IAB donor-controlled mechanism or IAB node-basedmechanism.

Wireless backhaul links are vulnerable to blockage, e.g., due to movingobjects such as vehicles, due to seasonal changes (foliage), or due toinfrastructure changes (new buildings). And also, traffic variations cancreate uneven load distribution on wireless backhaul links leading tolocal link or node congestion. Thus, the topology adaptation should takeinto account the current network performances such as channel quality,number of hops (latency), load information etc. On the other hand, someof the information is already available on F1-AP, e.g., the measurementreport, so it's expected that the IAB donor should get hold of suchinformation since it has CU-CP entity. In addition, the IAB donor hasindividual RRC connections towards each IAB node (i.e., the MT in IABnode). So, it's not difficult for the IAB donor to acquire the detailedinformation from each IAB node (through the MT entity in) and UEs. Also,the routing will be determined by the information in the IAB node. Inthis sense, the IAB donor-controlled topology adaptation, i.e., withhandover, could be the baseline.

Proposal 1: RAN2 should agree that the existing handover procedure isthe baseline of topology adaptation, assuming the IAB donor has enoughinformation for routing.

2.2. Issue in IAB Node in Setup and Operation Phases

In this section, the issues in initial phase are considered from IABnode power-on to in operation.

At the power-on of an IAB node, the MT entity of IAB node access a cellto connect to OAM and to acquire some information for nodediscovery/selection, as captured under step 1 of “Integration of IABnode.” The MT entity needs to access IAB-capable cell (or hopefully IAB“donor”-capable cell) since it's expected to be handed over to asuitable IAB donor/node at the next step, if Proposal 1 is agreeable.One of simple solution would be to have the SIB Indication.

Proposal 2: RAN2 should discuss if the cell should indicate whether theMT entity is allowed to access, e.g., with a SIB Indication.

At the setup of the IAB node, the MT entity will receive necessaryconfiguration to establish e.g., F1 connection, PDU session, andtopology/route etc., as captured under step 2 of “Integration of IABnode.” With regard to the topology/route management, it could be assumedthat it's a bearer configuration for relaying and done by RRCReconfiguration, if Proposal 1 is agreeable.

After these setup, the IAB node starts providing service to UEs or otherIAB nodes, as captured under step 3 of “Integration of IAB node.”

Proposal 3: RAN2 should discuss if RRC Reconfiguration that contains IABspecific configuration, e.g., bearer configuration for relaying,completes the setup of IAB node.

If Proposal 3 is agreeable, it's questionable how the gNB knows whetherthe MT entity desires to operate as IAB node since “An IAB node canfollow the same initial access procedure as an UE” in the initialaccess, i.e., it's unclear how to determine whether IAB specificreconfiguration is needed. In this sense, the MT entity should informthe gNB of its preference to operate as IAB node.

Proposal 4: RAN2 should discuss whether the MT entity is allowed toinform the NW of its preference with regard to IAB node operation.

In some cases, the NW may reject or release RRC connection towards theMT entity. Unless some IAB specific case value is included, the MTentity will retry the access to the cell (e.g., after the same wait timeas normal UEs). However, if no suitable parent node is available forexample, the MT entity should consider some other way around to getintegrated in an appropriate IAB topology (e.g., waiting for a longertime). It may depend on deployment strategies and NW implementations, soit's helpful for IAB node implementation to discuss what the expectedbehavior is.

Proposal 5: RAN2 should discuss whether there is IAB specific reason torelease/reject RRC connection, and if any, whether to have new causevalues.

At the topology adaptation in operation phase, there are severalscenarios, and some issues are observed in case to support inter-gNB (orinter-CU) topology adaptation. If the IAB node in service is handed overto another gNB, it will need the context transfer of not only the IABnode (i.e., its MT entity) but also all IAB nodes and UEs that areserved by the IAB node. Since it's a kind of group mobility scenario, itmay need some Xn signaling enhancements if it's handled efficiently. Inaddition, F1 connection re-establishment may be necessary in case of theinter-CU scenario.

Proposal 6: RAN2 should discuss whether to support the inter-gNB and/orinter-CU topology adaptation, that may require the context transfer ofnot only the IAB node but also the served IAB nodes and UEs.

Similar issue could be considered in case of RLF within IAB topology.Upon RLF, the MT entity will select a suitable cell and initiates RRCRe-establishment, staying in RRC Connected. It's useful to maintain theservice continuity to the UEs, to fulfil the requirement “withoutdiscontinuing services for UEs.” However, if the MT entity selects anIAB donor or an IAB nodes that is integrated within another IAB topologybelonging to different IAB donor, then the same problem in Proposal 4occurs upon the context fetch procedure. To ensure the re-establishmentto the same IAB topology that the IAB node was previously integratedwith, the MT entity should be informed of the IAB topology that thereselecting cell is belonging to. For example, the cell may broadcast an“IAB topology ID” and the MT entity takes it into account upon there-establishment procedure.

Proposal 7: RAN2 should discuss whether the MT entity should take IABtopology identity into account upon RRC Re-establishment.

Appendix

1. Introduction

RAN Plenary approved the study on Integrated Access and Backhaul for NR.One of challenges in this study is the topology management and routeselection in response to support of multi-hop networking. The TRcaptures the requirement for the topology adaptation.

5.2.2 Topology Adaptation

Wireless backhaul links are vulnerable to blockage, e.g., due to movingobjects such as vehicles, due to seasonal changes (foliage), or due toinfrastructure changes (new buildings). Such vulnerability also appliesto physically stationary IAB-nodes. Also, traffic variations can createuneven load distribution on wireless backhaul links leading to locallink or node congestion.

Topology adaptation refers to procedures that autonomously reconfigurethe backhaul network under circumstances such as blockage or localcongestion without discontinuing services for UEs.

Requirement: Topology adaptation for physically fixed relays shall besupported to enable robust operation, e.g., mitigate blockage and loadvariation on backhaul links.

As FIG. 25, RAN3 identified the three scenarios for IAB failure recoveryas part of route management. In FIG. 25, node A1 and node A2 are the IABdonor nodes, others are IAB nodes. Also, in FIG. 25, the dash linerepresents the established connection between two nodes, the arrowrepresents the established route after failure, and the solid linerepresents the new established connection.

In this appendix, the RLF recovery involving RRC Re-establishment, i.e.,especially for the scenarios 2 and 3 in SA deployment, is discussed fromRAN2 point of view.

2. Discussion

2.1. Scenario 1

Comparing the three scenarios that RAN3 identified [3], one ofdifferences is whether the IAB node has a redundant link that is alreadyestablished. In scenario 1, the IAB node that experiences RLF, i.e.,“C”, potentially only needs to re-routes U-plane path, i.e., via “E”.

In NSA deployment, RLF of the primary link on IAB backhaul is expectedto be handled by MCG, i.e., MeNB, with SCG RLF as it is today, wherebyit does not involve RRC Re-establishment.

In SA deployment, RLF of primary link could need to change of Pcell. Ifthe packet duplication is configured for SRB, the RRC message reachesthe IAB node via the secondary link.

So, the topology management due to RLF could be handled by the network.

Observation 1: The failure scenario 1 may be handled by the network withthe existing functionality.

2.2. Scenario 2

Scenario 2 does not assume that the problematic IAB node has anyredundant route that is already established.

In NSA deployment, the IAB node that experiences SCG RLF needs toestablish a new link, whereby it could be handled by MeNB with e.g., MNinitiated SN Change via MCG.

On the other hand in SA deployment, the IAB node that experiences RLFneeds to select a suitable cell and then initiates RRC re-establishment,i.e., towards “F”, if the current UE behavior is applied.

Observation 2: In SA deployment, the failure scenario 2 will need tohave the IAB node to initiate RRC Re-establishment.

In this case, it's not clear how the IAB node selects the suitable cellin the expected IAB topology, since the current cell reselectionrules/parameters do not assume IAB backhaul RLF recovery but considernormal UEs. For example, the IAB-related configuration may be reset ifthe IAB node reselects a cell on non-IAB layer that has higher frequencypriority. While it may not a problem in the setup phase, i.e., the firstintegration to the IAB topology, it's not the same for RLF case since itwill happen after the IAB operation starts, i.e., the IAB node alreadyserves its downstream IAB nodes and UEs.

Proposal 1: RAN2 should discuss how the IAB node selects a suitableparent node, upon RLF.

If Proposal 1 is agreeable, it could be expected there need somethingdifferent from/prioritized over the existing cell reselection procedurefor normal UEs. In case of the initial access of IAB node, it followsthe current UE behavior since it acts as a normal UE. On the other hand,such a new mechanism should be applicable to the IAB node already inoperation. In other words, it could be applied upon RLF.

Proposal 2: RAN2 should agree to define an additional cell reselectionrule that is applicable only to the IAB node upon RLF.

If Proposal 2 is acceptable, some options could be considered asfollows.

-   -   Option 1: The IAB node reselects a cell that broadcasts the SIB        Indication, “IAB Support”.    -   Option 2: The IAB node reselects a cell that is indicated by the        network.

Option 1 is simple UE-based mechanism, but it's not sure whether the IABnode can reselect a cell integrated within the suitable IAB topology.

Option 2 is a NW-controlled mechanism, whereby the information ofsuitable cell(s) is provided by the network. For example, the IAB nodereceives the information in RRC Connected by OAM or RRC (via dedicatedsignaling). Upon RLF, the IAB nodes prioritized the cell(s) based on theinformation, even if e.g., the other cell is the best ranked or hashigher absolute priority. The details would be FFS.

Proposal 3: RAN2 should agree that the information of suitable cell(s)for reselection upon RLF of IAB node is provided by the network.

2.3. Scenario 3

Scenario 3 is similar to Scenario 2, but the IAB node that experiencesRLF is assumed to re-establish its RRC connection in different IABtopology, i.e., towards different IAB donor or CU. While it's up to RAN3to decide whether the inter-CU topology adaptation, it could be assumedbetter that the IAB node stays in the same IAB topology after RRCRe-establishment, i.e., aiming to connect with “F”. It means Proposal 3above is still applicable to Scenario 3, but if the IAB node cannot findany suitable cell, i.e., within the suitable IAB topology, at the endthen it should be allowed to reselect any cell.

Observation 3: Failure scenario 3 might be assumed as an exceptionalcase of Failure scenario 2.

1. A method executed by a relay apparatus configured to: connect to afirst upper apparatus; and have a function of relaying communicationbetween the first upper apparatus and a lower apparatus, the methodcomprising: detecting a connection failure with the first upperapparatus after starting the relaying; transmitting a reconnectionrequest to a second upper apparatus which is a candidate of areconnection destination of the relay apparatus in response to detectionof the connection failure; receiving, from the second upper apparatus,in response to transmission of the reconnection request, a radioresource control, RRC, setup message indicating that not thereconnection but an RRC connection establishment has to be performed;and transmitting, to the second upper apparatus, information on behaviorof the relay apparatus in response to reception of the RRC setupmessage.
 2. The method according to claim 1, further comprisingreceiving system information from the second upper apparatus, whereinthe system information includes information indicating whether thesecond upper apparatus support the relay apparatus.
 3. The methodaccording to claim 1, further comprising: receiving, from the firstupper apparatus, information indicating candidate upper apparatuseswhich are candidates for a reconnection destination of the relayapparatus; in response to the detection of the connection failure,determining the second upper apparatus among from the candidate upperapparatuses based on the information.
 4. An apparatus for controlling arelay apparatus configured to: connect to a first upper apparatus; andhave a function of relaying communication between the first upperapparatus and a lower apparatus, the apparatus comprising a processorand a memory, the processor configured to execute processes of:detecting a connection failure with the first upper apparatus afterstarting the relaying; transmitting a reconnection request to a secondupper apparatus which is a candidate of a reconnection destination ofthe relay apparatus in response to detection of the connection failure;receiving, from the second upper apparatus, in response to transmissionof the reconnection request, a radio resource control, RRC, setupmessage indicating that not the reconnection but an RRC connectionestablishment has to be performed; and transmitting, to the second upperapparatus, information on behavior of the relay apparatus in response toreception of the RRC setup message.
 5. A relay apparatus configured to:connect to a first upper apparatus; and have a function of relayingcommunication between the first upper apparatus and a lower apparatus,the relay apparatus comprising a processor and a memory, the processorconfigured to execute processes of: detecting a connection failure withthe first upper apparatus after starting the relaying; transmitting areconnection request to second upper apparatus which is a candidate of areconnection destination of the relay apparatus in response to detectionof the connection failure; receiving, from the second upper apparatus,in response to transmission of the reconnection request, a radioresource control, RRC, setup message indicating that not thereconnection but an RRC connection establishment has to be performed;and transmitting, to the second upper apparatus, information on behaviorof the relay apparatus in response to reception of the RRC setupmessage.